Ash was 10 in the OS, he was 10 in AG, he was 10 in DP, he was 10 in BW, and he's 10 in XY: whether you like it or not, this is the only fact here, I'm sorry.
Ash's Bulbapedia article has always stated that his age is 10, but it only used EP001 as the reference, because his age hadn't been mentioned again, until BW001. It was until then that we could confirm that Ash really stayed 10 after all. Until then, it was
assumed that he had grown more, as the passage of time had been clarified during the length of the series, and his character was developed further. We know from what the narrator says that it took around a year for Ash to get all his Kanto gym badges for example and that he had to wait a couple of months to compete in the Indigo League. I don't know about you, but I even noticed that he got a bit taller. In OS, Misty was a few inches taller than him, but when she reappears in AG, Ash is now the same height. From things like that, the assumption became more strong.
I'd dare to say (and I think I said that in a previous post) that when BW began and Ash's age was officially clarified again (for the first time in almost 14 years [!] by the way), his character was drawn shorter. Perhaps it's just a side effect of the new art style, or maybe I need glasses :I
After so many years, you expect some growth after all.
Also, I don't think that Ash character began his change in DP. The series overall feel did changed through DP, but it wasn't that immediate. It was a long process that began at the end of Battle Frontier. That's why I say that DP is at the half of the scale between kodomo and shonen. I'd say that Ash's personality matured through DP. And I keep saying "matured" because I think it wasn't obvious at that time that he was still 10, because of the assumption that I'm mentioning above. If Ash never really grew after all, at least he had become smarter and a higher quality trainer. In my opinion, BW was when Ash's character was changed. I say that it was rebooted because he began to act like he did at the beginning of OS. Seeing him acting like that again was very painful to me, because of all what he had achieved up to that point.
But well, now we are all sure that he was 10 years old all the time, and that he'll possibly be 10 years old for the rest of the series, and I'll have to accept that. No matter how much I'd like to see Ash growing up and accomplishing his goals, I'm not the writer, so I can't make it happen.
