My son used to hand flap, but since he's been learning to play piano he's started drumming his fingers instead. What's interesting is that he's not just "drumming", he's playing broken chords, or practising phrases of music he's learning.
He's obviously obsessed with music, and his stimming is in fact helping him to progress faster at his chosen subject. But if he doesn't have a flat surface to practise on, he'll resort to hand flapping. I don't mind that so much... well, I'm thirty eight, and I'm still at it.
I think it gets more contained, and managable as you get older.