If there is no proof, it does not matter if it is legal/illegal. It probably cannot be prosecuted, especially if it happened in the "past".

I don't think it can be abuse, there's no form of coercion possible online that the minor can't walk away from. That's not to say it may not be hard for the minor to want to do so, just that it is possible whereas with abuse of a parent/child nature the child has no other place to go live, no one else who will feed him, etc. With cybersex the child can keep away from the internet, tell an adult they trust, etc. It does not make it right/legal from the adult's side that the minor does not disengage, and if it is caught by other adults in a timely fashion and provable, the adult will probably be prosecuted.

Whether it has negative effects into adulthood depends on how the minor processes the experience. A minor is not in the position to "understand" sex yet and its healthy position in life and early experiences of sex can skew subsequent knowledge and experiences.