To be honest I believe the whole 10yo meme has more to do with Ash than with the games :DThe game protags are not 10.
They have never been 10. The only confirmed ages are 11 and 12, and some protags look like teenagers.
You do not need to be 10 to be a trainer. Not only is this never stated in the games, you can fight friggin toddlers in a couple.
I hate it when people perpetuate the 10-year-old thing.