Apparently 80 per cent of children age five and under are active internet users, according to a recent study. So that's where all those illiterate YouTube comments come from!
Snark aside, there's nothing inherently wrong with kids that young being online, so long as what they're watching is heavily supervised. And given the motor abilities of your basic pre-schooler, I'm guessing they're not typing in too many URLs on their own. [Mashable]