Actually, that's true in the original japanese version of BW as well. The dub is not to blame in this case. Heck, they actually did try to show he ages in the first season. When they return to Viridian City, the dub says it's been a year since they returned, but the original japanese script just says "it's been a while", and nothing else specific.

God I hate this joke…

Look at these three Ash's, one from the first day of his journey, one from Sinnoh, and the most recent iteration. Look how his legs and arms get longer, how he gets thinner. The kid on the left is clearly 10 years old. But the new Ash is difinitely much older. I would have no problem thinking the new Ash is sixteen. And anyone who wants to say the beginning of Black and White says he's ten, that was the dub only. And the Dub also said fire types beat water types.