Pretty much as soon as they can steer the sled or 4-wheeler. My neighbor's kid was 6 and he had a little junior model 4-wheeler he used to drive around the farm. Another buddy of mine's 9 year old daughter had a little Indy sled that she rode all over the place. I think she had been riding for a year or so before I met them.
The department of public safety says 8-11 year olds can operate a sled w/o having to take the safety course, but only under direct adult supervision. That means kids as yound a 6 or 7 probabbly know how to ride...especially if you live in the Kingdom or in Addison county.