the general "creepiness" rule I've heard is half your age plus 7.
But... I say that if a 50 year old wants to date an 80 year old.. why not? They're both mature enough (hopefully) to be able to make such decisions for themselves.
Your relative was 17, and I personally don't think that most 17 year olds are experienced enough in the world to really weigh all sides of a decision. I am not saying that 17 year olds can't fall in love or can't make a commitment. I just think that before they formally follow through with anything (such as getting married,) they wait a couple years. I, for one, changed A LOT between the time I was 17 and the time I was 20.
Also, remember that there are many many marriages between people who aren't 20+ years apart that don't last.
