Meaning she isn't really allowed to make decisions on her own, she hasn't truly become an "adult." I'll be honest though, some people age and mature slower than others and some faster.
I think most 16 year olds aren't really ready to be in a serious relationship, and I think most 21 year olds who are interested in 16 year olds are interested because on some level (even if it isn't an intentional, thought out process) they recognize a 16 year old girl is probably much more easily manipulated than one their own age.
The 18 year old age limit to me is more of a recourse for angry parents than a real cutoff to protect children.
The parents shouldn't have let their 16-year-old daughter have a 21-year-old guy over the house if they didn't like the idea of a 21-year-old guy having sex with their daughter. If the daughter admitted to "making out", the reality is probably that he has violated her at least three different ways.
That said, it was about the maximum age gap without getting skeevy, IME.
The girls my age who were dating guys older than that were usually headed for trouble.
Most people would not think it strange at all if a 40 year old man was dating a 35 year old woman--5 years just isn't a huge gap.Until very recently that would apply to homosexual intercourse between parties of any age.I'm not really against the concept of statutory rape, but I do think that most civilized countries and many states set the age limit at around 16 for a reason.Anybody who's 21 (almost done with college) and dating a 16-year-old (still in high school) has some serious personality issues or they'd be dating someone closer to their own age/maturity/life experience level.Which seems to describe your friend to a T--"socially awkward and youngish." So remember, kids--instead of being flattered when you're in high school and someone college-age or older asks you out, think to yourself, "What is wrong with them that they're not dating people in their peer group?
(Possibly b/c that made the guys legal to drink, now that I think about it...) Although it's certainly possible that they are having intercourse, they may not be.