It has long been known to psychology that, during childhood, and adolescence, people mature emotionally and intellectually at an incredible rate.
But nobody ever really impresses upon kids that this process slows down dramatically in the late teens.
The result is that, like, 16 year-olds have vastly inflated expectations for the maturity of 20 year-olds. There's so much less difference in maturity between a 16 year-old and a 25 year-old than there is between a 16 year-old and a 12 year-old.
It seems to me that it is precisely this chasm between adolescent expectations and adult reality that gives relationships between the two such a high risk of harm.
At least, I'm pretty sure I would have been much less damaged by my the internet relationship I had during high school if I had understood that pretty much the only real difference between being 17 and being 22 is how intense your emotions feel.