Yeah, the age subject (with regards to relationships/sex) is pretty weird. A 15 year old with an 18 year old seems completely weird to me, but a 16 year old and an 18 year old sounds totally normal, even though it's possible for the 16 year old to only be 1 day older than the 15 year old.. it just sets off a switch in my head for some reason.
But then, 20 and 30 together sounds totally normal to me.
I personally have a real hard problem guessing girl's ages.. there have been lots of times I'm talking with a girl and just assume (based on the way she looks and talks) she's at least my age, only to find out later she's like 15 ( I'm 18 ) and be totally weirded out. Then there have been a few times where I was talking with someone who was 20 and I was positive she wasn't over 15 before she told me.
It seems to be getting harder and harder to determine too, with lots of young women these days developing very early. There are girls in my neighborhood that would always come over to play with my dog that, judging from their bodies (and how they flaunt them), you'd think were at least 16-18; but juding by how they acted (and the fact that none of them drive) you'd think they were like 10. I find out later that they're 12 and it's just damn creepy.
It's almost like you gotta ask for an ID before even saying a word
But, anyway, to answer the original question.. as an 18 year old, I can go as low as 16, or as high as around 25. If I were in my late 20s I'd say probably anything from 18 to low-mid 30s.