Ash is 16. He spent a year in kanto(10), then another year in Johto(11), honen(12) another year, then he went battle frontier(13), finally he spends a year in sinnoh(14), plus he is going to isshu he is (15)
A league takes place every 4 months, and in japan he is recognized as 5'5. this would conclude that (and this is the only logical reasoning) he id denying his age to hit on 10 year old girls. either that or he's 14/15.
ash is indeed 26 years old now but if u look at it clearly, he only looks 16-20 years old. I've been a fan since a young age. I've studied his height, weight, and his progress since he was a kid in Kanto. I answer this question with complete honesty. Ash Ketchum is indeed 18 years old. Question answered.
There are multiple ways of answering this question:
If going by the theory that every league is a year, including the Orange Archipelago(that everyone keeps forgetting) and the Battle Frontier, that would put him at 17 at the end of Unova.
Now, if we were going by actual years in the real world, he would be in his twenties. This in my opinion is the most unlikely. Assuming that Professor Oak is around the average height for a grown adult male in Kanto, even after the decrease in height that would usually come with aging he still has a decent height advantage on Ash, implying he most likely hasn't even hit full puberty. That doesn't mean it couldn't be a heredity issue causing him to remain short-considering we don't know the height of his currently non-existing father- but since his mother is his only known parental figure and she is fairly tall, we can assume he will end up tall as well.
Now, if we go by the earlier mentioned theory that leagues happen every four months, he would be a little over twelve. This would be a likely age based on how he acts and talks-including the minor high increase and voice change that could be implying he is hitting puberty-, but we have no confirmation on the length of the leagues.
Another option to look at would be going based on episodes. If we take every episode as a day and multiply a third of it by two to cover episodes that had more than one day in them, we would come up with 2.9 years worth of time having passed, which would make him about to turn 13. Also possible, but not the most likely since a majority of animes do not count the passing of time on their episode count -or even day count in the episodes- but you never know.
There is also a theory that legendaries (most say Dialga but I would say Ho-oh since it was the first legendary they came in contact with, and they haven't seemed to age much since) stopped them from aging. This is highly unlikely, both because the legendaries would have no reason to do so and there is no recorded case of it happening in the show, and for the fact that Ash would have acknowledged that fact at some point-no matter how oblivious he is.
Now-in my opinion- it is time for the most plausible option.
In many animes the creators will put in more episodes than possible in the span of a year yet no time passes. This is usually either because for the plot to work out correctly the character must stay in the same grade or location which wouldn't happen if time passed(Detective Conan being a good example) or in cases like Pokemon where they focus the story around a child as the main, but don't wish to change the main so they just don't age said main.
So in conclusion, chances are, Ash is still ten because of the creators.
Late Kanto 11 (a year later when he arrives back at Viridian City)
Unova 10... (as stated in "Shadow of Zekrom" BW001)
Assuming every episode is a week, every season is a year, he should be 25.
By using the amount of birthdays he has then he is still 10...
It states in the first episode he is 10.
He then says, "It's been almost a year since I've been here" or something like that when he returns to Viridian City. (So he's 11)
In late Hoenn his voice breaks (lol, jks change of voices)
After that I don't think it says anything until Unova, in "In the Shadow of Zekrom" the narrator says he's 10.
If we were to assume a logical inference the each episode in the Pokemon series represents a day, there are 804 episodes so from this point on this should be recognized as a representation of 804 days(dy) in Ash Ketchum's life. Also assuming each movie represents about 3dys, with 15 movies multiplied by 3dys it equals 45dys represented. So add that to your original 804dys, and your total dys represented is 849dys. There are 365.25 days(dy) in a year(yr), 849dys divided by 365.25dy equals 2.32yrs. Ash Ketchum turned 10yrs old on the first episode of the series which would technically make him 12.32yrs old, logically rounding up from so many possible unaccountable days within the episodes and movies, Ash would be between 12.32 years old and 14 years old. There you go.
ash spent 2 years (3 if watching dub) in kanto. johto and hoenn = 1 year, sinnoh = -1 years due to encounters with dialga. unova = 2 years (3 if ovas counted) at the beginning of of kalos he is 15/16/17