I started at 6 on skis. Like almost every Canadian kid I was put in hockey very young and hated it. Number one I'm not a people person so team sports suck. Number two who wants to get up at 6:00 on a Saturday morning to go play hockey? Not me when I was 5, that's who (not)!
So my dad said "You either ski or you play hockey"... And 26 years later my dad and I still meet up on the hill and go skiing/boarding together!
I switched from two planks to one in grade 6 when my school offered a course. One or two days of sore knees and ass and I was hooked. Wish I had started on a board earlier but in 1986 it was still almost non-existant in southern Ontario!
I've seen little tiny kids having a ball on a board. I say if they're willing, start them young!
+1 to starting on a board instead of skis. Can't stand the pizza/snow plow pylons all over the hill heading straight down. A snowboard forces you to learn to turn, or you'll end up heelsliding all day. Kids love speed usually so they'll stop heelsliding quicker than an adult will.