I can palm it easily. Probably from 9th or 10th grade onward (finally stopped growing in the 9th grade after a crazy 8th grade growth spurt)
Just measured - stretched my left hand out and it comes in between 9.5 - 9.75. And oddly enough, when I measured my other hand it was a lower number - roughly 9. That explains why I've always had an easier time palming a basketball with my left hand.
So much wider than my hand! No wonder I can barely palm the ball.