I agree with Loren...My daughter is 6 and she already tries to pick out what she wants to wear. I say that as long as it matches and is appropriate for whatever event you are going to, it should be ok for him or her to pick out what they wear. Doesn't matter the age. I allow my daughter to pick out her clothes but it is all within reason.

I say whenever they ask, its not a big deal and it makesthm feeel like a "big kid".