Does the offender have to be an adult for it to be considered child molestation?

Depends: We understand the concept of child to child exploratory play may include exposure of genitals. "you show me yours, i'll show you mine". By convention we use this lable if the kids are close to the same age (within 2 years). If you get older kids exploring younger it moves into the area of predatory behavior. The bridgebetween cps/the legal system etc may be better defined in your local.

State laws vary: You need to find out the statutes in your specific state in order to know the answer to this question.