I dated a few 21 year old guys when I was 16, and by senior year of high school (17) I was in love with a guy who was 22. We stayed together for over 2 years and were very serious. None of these relationships got physically intimate until after I graduated high school and turned 18. It seemed like boys my own age were children, and though I tried going out with a couple of them, it was far less personally meaningful and far more about the "couple" image we projected at school.
Funny, when I first read this thread, I was alarmed and taken aback, coming at it from the perspective of a parent. Life can really come back and kick you in the butt sometimes!
My final thought though, is that they should give him a couple years and then see if a romantic relationship is something they both want. If it's meant to be, it can wait.