User blog comment:Alanomaly/Top 10 Ben 10 Plot Holes/@comment-6883075-20160412120445/@comment-1478058-20160412194958

Actually Ash is *technically* still 10.

However I believe that was only in the english dub of Black and White that stated he was still 10. The Japanese version hasn't discussed Ash's age or any "years" passing since Episode 1 of Kanto. So in the Japanese version Ash could very well be 13 or 14, but I doubt they will ever directly state his age.

TL;DR: Ash isn't 11. He's technically older then 10 but not an adult yet (so he could be anywhere from 10 to 17 years old atm).