Short answer is when they're ready, and that is around four for most children.
Ski schools generally maintain that below the age of four, kids simply don't have the muscles or attention span for a structured, day-long program. Around the mid-Atlantic, most ski schools will take kids four and older. Check out local ski schools for programs that take childcare a step further and get the little ones out on the snow. Most of the programs allow a toddler to opt out if he or she doesn't want to go out into the snow. For those who want to play in the snow, the time outside is about fun with some subtle instruction and technique introduced.