It's a plot hole. The narrator upon the start of Best Wishes/Black and White still said he was 10. And they movie you are alluding to is Movie 3 - Spell of the Unknown. I believe when they were nearing Viridian City it also mentioned about a year or a year has pasted since they were their last. Then the training for the league is implied like 2-3 months. So clearly normal age terms don't apply to anime.