According to a child abuse and reporting workshop I just attended, the only sexual activity that is allowed is between two 13 year olds. If they are 13 and 14, it is reportable. Like I said, I think this is problematic but understand why it exists.

I'm pretty sure that sex between a 16 year old and a 50 year old would be considered statutory rape. If cybersex, then there is probably some other term like lewd and lascivious.

In terms of what is required for people not just therapists to report, even if something is in the past, if the adult is currently in contact with minors or there is reasonable suspicion that that is the case, it is mandated reporting. What happens after that is an investigation by an agency, that does not necessarily involve police. Nothing may come of it, true, but it is still considered abuse and still mandated to be reported.