A lot of this depends on the child.  In general I would say around 18 months, but if your child tends to put things in his/her mouth, ears, or nose I would suggest waiting a little bit longer.

When you do decide to give it a try use the non-toxic brands, this way if a piece does make it to the mouth it wont be that bad.

Only allow your child to use the clay with an adult that is paying very close attention, just like with anything else you never know what could happen.