Here's a great blog post I found on it: Part 1 Part 2
Some advice I've been given:
Old enough to walk is not necessarily old enough to ski. They need to be able to follow instructions, as skiing is not an instinctive activity. You can ski around the kiddy hill with them between your legs if you want, but I've been told by other parents that before about age 3, if you take your kids skiing, you're just doing it for yourself, not for the kids. For these reasons, and the advice in the blog post I copied, I'm wainting until after my daughter turns 3 this winter if she seems mature and patient enough to follow my instructions. But if you take your kid earlier, and have a good experience. Please share here.