Note: Black & White has to to be considered a separate series, not correctly connected to the main anime (HERE for clarification). However, in this article there are some references which mention the discrepancies present inside the series.
We can’t start talking about Ash without having clarified first the question about his age.
As mentioned in several previously published articles, one of which you can read at THIS LINK, the urban legend that Ash is always 10 after many series, has been largely disproved by several factors, like official sources from the creator of the animated series and many informations given in the anime episodes themself, such as:
> The first animated series, in which OFFICIALLY has passed more than a year: in the various episodes were regularly inserted tips about the passage of months. Therefore, you can know with absolute certainty the period of time of each episode.
> The episode in the Rival Destinies series “Goodbye, Junior Cup – Hello Adventure!”, where Dawn clearly says that a year has passed since her participation to the previous Wallace Cup, so it’s impossible that Ash was nine in that series.
> Already the elapsed months that were mentioned in the episodes (two monts after Ash’s departure in the episode “The School of Hard Knocks“, two months before the Indigo League in the episode “Showdown at the Po-ké Corral“, one month before the Wirl Cup in the episode “Around the Whirlpool“, three months before the Silver Conference in the episode “Why? Wynaut!“, three months before the Hoenn League in the episode “Eight Ain’t Enough“, one month before the Kanto Grand Festival in the episode “New Plot, Odd Lot!“, one month before the Sinnoh League in the episode “The Eighth Wonder of the Sinnoh World!” and two months before the Unova League) occupied in total more than ONE YEAR in the timeline of Pokémon anime.
> The third mini-movie (Pikachu and Pichu Brothers), where Ash celebrated the anniversary of his meeting with Pikachu. It was not the first anniversary and the movie must have took place between the 4th and the 5th season (for reasons you will soon read) but anyway there is NO DOUBT ABOUT IT: since Ash’s departure, more than a year has passed.
> The growth of Ash’s Lapras, who changed in appearance in one of the last episodes of “Master Quest”.
> In the book This is Animation Pocket Monsters The Movie “Mewtwo Strikes Back” “Pikachu’s vacation” Takeshi Shudo explains that between Mewtwo’escape from the Team Rocket’s base and the moment in which Ash received the invitation for New Islan, a year passes. Anime writers have decided to make pass only a couple of months (since Mewtwo’s escape is present in the episode titled “Showdown at the Poké-Corral“), thus proving yet another mistake concerning the timeline in the anime. The film “Mewtwo Strikes Back“, indeed, has a precise collocation in the anime: the events took place between the episodes “The Pi-Kahuna” and “Make Room for Gloom“.
> Several trainers visibly younger than Ash, have already obtained their first Pokémon (Including Trevor and Scottie, in recent episodes of XY), but, as you know, it is NOT possible to receive the first Pokémon from a Professor or affiliates BEFORE having turned 10 years old.
> In the episode “A Goldenrod Opportunity” (Pokémon Johto League Champions), the Magnet Train was not built and it needed a year to be completed. In the Best Whishes episoded where Iris and Cilan decide to travel from Kanto to Johto, they decide to use that same train, showing even in this case the passage of years in the series.
Also, in the recent episode “A Festival Trade! A Festival Farewell?” of the XY series, Jessie makes a reference about her birthday (in the Japanese version too) that was even celebrated during the same XY series. Demonstrating once more that, in the story, there is a passage of years.
More reasons in the article I linked above.
As for the official information given by the creator of the animated series, we are going to open a broader case concerning the age of Ash in the various series.
Takeshi Shudo, creator of Pokémon Anime, published two books in 1997 and in 1999 that unveiled some of the secrets of the animated series (if you are curious about it you may check THIS LINK for a detailed summary of the books), including the truth about the day of Ash’s departure on his journey.
In Pallet Town it existed the rule that new trainers could begin their formative journey on APRIL 1st following their tenth birthday. This fixes the inconsinstency in the first episode of the animated series in which it’s said that Ash was leaving because he just turned 10, but at the same time Gary and other two trainers left with him. Was it everyone’s birthday, perhaps? No, simply the writers have made a mistake.
Since during the 9th episode of the animated series it was made clear that two months have passed since Ash’s departure, here’s another confirmation that he has been 10 years old only in the first 6-7 episodes of the animated series.
For what concerns the other series:
In the Orange League Ash was 12. Summer was confirmed in that period (the 2° movie). The previous summer (the one in which Ash was 11) was in the period of the censored episode “Beauty and the Beach”. As already mentioned, in the first series were inserted specific references to the months in which the episodes were set. Ash left for his journey in April, and after 9 episodes May arrives. We clearly understand that the episode “Beauty and the Beach” was set in the adjacent summer.
After several episodes (enough time to make 6 months pass) the events of “Holiday Hi-Jynx” took place (which, according to the OFFICIAL numeration, it’s the episode 39) and an episode of the New Year’s Eve titled: “It’s New Year’s Eve! Pocket Monsters Encore” but which has never been broadcasted because of the scandal of the episode titled “Electric Soldier Porygon“.
Another handful of episodes later we have “Princess vs. Princess” (which was postponed due to “Electric Soldier Porygon“, but remains the fact that according to the official numeration it’s the episode 50) set on March 3rd because it refers to the celebration of Hinamatsuri. Five episodes later the events of “The purr-fect hero” (this episode was also postponed) were set on May 5th because it refers to the Children’s Day in Japan. Here, a year has already passed. In the following episodes, it’s further confirmed that it was Summer (the episode “Riddle Me This“, “The Breeding Center Secret“, etc.). From there to the Indigo League another two months has passed, so the Indigo League was held in the month of July, while the Orange League was set around August/September.
Together with Hideki Sonoda, Takeshi Shudo also wrote a novel dedicated to the film “Mewtwo Strikes Back” in which he explained that between Mewtwo’s escape from the Team Rocket’s base and the moment in which Ash received the invitation for New Islan, a year passes. Anime writers, instead, have decided to make pass only a couple of months and this way they made another mistake concerning the anime timeline.
It is not clear, however, whether this year should be considered as an additional year which has to be added to the timeline or Shudo wanted to collocate Mewtwo’s escape and film in the same year in which the first series takes place.
During the Johto League Tournament Ash was about 12/13 because Winter was confirmed during the Johto League Champions series (episode “Moving pictures”). Two Christmas episodes of the Pokémon Chronicles series are also set in this period (consequently the episodes in The Johto Journey series were set during Autumn). The League was held in the Spring/Summer of the same year.
During the Hoenn League Tournament, Ash was more ore less 13 because the Advanced and Advanced Challenge series took place right after the Johto League (in some episodes of the two above mentioned series it has been said that it was still Summer). The Hoenn league was held in the following Winter (Source: the episode “The Relicanth really can!” set more than 3 months before the league, where the beaches were crowded with people, because it was Summer.)
References: these two pictures as a confirmation
Note: The Hoenn region is the most southerly located region and is based upon the Kyushu island (Japan), where there is an almost tropical climate. It may also be noticed in the anime during the episodes of the league, in which, although it was winter, it didn’t seem to be cold.
In addition, during this generation Ash has also reached Misty in height.
In Battle Frontier Ash was 14.
Ash returns to travel in the Kanto region, where his previous travel lasted one year. The series starts more or less during the Winter, in fact the episode “Time Warp Heals All Wounds” is set on the Christmas period, as in the Japanese air of it.
Later, in the episode “Spontaneous Combusken” it’s shown that it was again Summer, as was in the movie “Pokémon Ranger and the Temple of the Sea“.
The last episode of the Battle Frontier series is clearly set on February, because in the Kanto region during this month the sun sets at around 5 PM, just as shown in the episode (further confirmation comes from the DP series which takes place in Spring). Since almost an year is needed to cross the Kanto region, this is the reason why the same amount of time has passed in this case too.
During the Sinnoh League Tournament Ash was 15. This was because in Diamond and Pearl series it was Spring and in Battle Dimension and Galactic Battles it was Summer. (The episodes of the Pokémon Summer Academy and the 12th movie were clearly set in the Summer). The league was held in Autumn (the tournament has started two months after Ash won the last badge, and here there are also confirmations related to the time the sun was setting).
At the end of the Sinnoh League, Ash stayed at home for a few months. He left for Unova in Spring (pink Deerlings) and so he was still 15.
Warning: The Black&White series should be considered separately, however, given its multiple contradictions and given the presence of it between two continuing series, we keep still account its timeline.
In Best Wishes 2 (Rival Destinies and Adventures in Unova), it was Summer (green Deerlings), so he just turned 16. The league was held two months after Ash won the last badge. Towards the end of Best Wishes DA (Adventures in Unova and elsewhere) it has been clearly said that Autumn was coming (in the episode “Butterfree and Me” we can see many migrating Pokémon).
Obviously, the beginning of the XY series is set in the late Summer- early Autumn (Summer Camp episodes are set in the mid-September period, exactly like in real-life). This is confirmed by the presence of a vineyard with ripe grapes in the episode “A Jolting Switcheroo!“. Subsequently is Autumn as we can notice from the presence of many autumnal landscapes and falling leaves, as in the episode XY073: “A Fashionable Battle!“.
The start of the XY&Z series is set in November because in the episode XY097 “Dream a little dream from me!” the protagonists are watching the Litleonids. Litleonids are a meteor shower mentioned in the Delta Episode of the OR AS games. It takes the name from the Leonids shower, which always falls in November (intentionally coinciding with the Ruby and Sapphire games release, reference also mentioned in OR AS). The same XY097 episode aired in Japan during November. The end of the series seems to be set in the spring and Ash is 16 and a half.
For obvious and logical reasons the Pokémon anime is terminated. Anyway if you are continuing to follow it, you must always take into account the instructions in this article and differentiate the Ash that we have always known (and we’ve all seen it grow) by purely commercial reboot.
A closer look at how Ash has changed and matured over the years at THIS LINK.