I suspect this has to do with the qvasi-perfect uniform distribution of a large portfolio of stocks. If you get monkeys to throw darts at a chart (and pick stocks randomly), you'd get wins =~ loses so ignoring commissions and fees, they'd break even. DOING A BILLION TRADES A DAY!
Now if these "young stars" backed by multi-billion VCs actually make above commissions, that's a mystery. Even if they do and it's not spectacular, then it has a lot more in common with plain luck and a bull market than actual wits.
I just don't buy the myth of the teenage superstar trader.