I have no idea which part of the world you live, but here nothing of that is true most of the time.

That would be a love story between a first year and a last year student. Maybe unusual, but mostly doomed to last 3 months at best.

Indeed a 17-19 is more common, and 18-19 quite common, but I wouldnt worry too much. Monitor yes, specially if the 16yo is somebody I care for/of.

But child abuse? That's a bit stretched.