I'm sure it varies from country to country. But no, the mandatory reporting thing isn't only triggered by things where another child is involved. It can happen anytime someone is a mandatory reporter. (In fact, during the situation I mentioned above, the adoption agency suggested I check with my original T to see if she'd ever reported it. She answered that she hadn't because she wasn't a mandatory reporter at the time.)

I think you're right that not all reported incidents from decades before will be pursued in any significant way. At the very least, though, they'll be down on paper somewhere, which might be important if additional/more recent activity is reported. In the end, just because there aren't any/anymore minor children in an abuser's home doesn't mean s/he won't find a target elsewhere.

I'm with Jacq...definitely best to figure out whether T is a mandatory reporter or not before s/he needs to go down the reporting path against your will. Maybe nothing would happen, but who needs the stress of worrying about whether or not it will if that's a concern for you?