How old can a child be to start wearing eye contacts?

Almost any age. Depending on the circumstances, age is not the primary factor to deciding about contact lens wear. Sometimes it can be easier on the child (and parents) to uses contacts than to struggle with glasses. This is especially true with young children. Speak with an eye doctor about the possibilty of contacts for your child. It can be labor intensive, but can work very well.
Depends. It really depends on the maturity level of the child. Improper lens care, poor hygiene, and overwear can lead to permanent vision loss. I would only allow under very close supervision and would not allow extended wear (sleeping in lenses).