My husbands' cousins' wife runs a dayhome and we let the 2-3 year olds handle the corn snake with supervision every once in a while.
They are to wash their hands before and after and they are to be really *gentle*.
This seems to go over really well and they enjoy it a lot.
It think that it is best to start early as a 6 year old will already have a reaction/response ingrained in their heads from their parents.
The 2-3 year olds are WAY better with the snake than most of the older kids, who shreek and say "gross" and think that the snake is slimy and is going to bite them. (brainwashing starts early...)
Its best to start early IMHO...