Mostly so that by the time you reach your peak physical ability in your 20's, you'll have sufficient practice to utilize it. It's not that you can't learn it,

Mostly so that by the time you reach your peak physical ability in your 20's, you'll have sufficient practice to utilize it. It's not that you can't learn it, you just won't be "great" at it when measured against others who started much earlier. Other factors are at play too. Like if you're not making money by playing the piano by early adulthood, you'll be spending your work hours doing something else. Where your "competition" will be playing the piano 24x7.