My oldest son and I started together when he was 8. He's already better than I am at jumps and grabs. My middle son started around age 5, though I haven't pushed him much and he's only been a few times since then (now age 6). He still can't link turns, but he's able to go straight and do j-turns to stop or slow down (mostly heelside) on gentle green trails. My youngest son tried skiing at 3, snowboarding at 4, but doesn't have the coordination to progress fast yet. He's now 5 and still working on turns.
Wow. On the hill at 3! That's cool.
I can imagine the advantage someone who starts so young would have over someone who starts in their 20's like I did. Their ceiling will be much higher.