I would have thought Carolyn was at least 18, though. I assumed she was already through with high school, which she could be but probably wouldn't be at 17. And, I had assumed she knew Joe through school and figured he was at most a couple of years older than she was, but then, I guess she did kind of go for "older men"!!
That's in the DS Bible too. Carolyn does go to the local public high school like everyone else and that is where she meets Joe.
What was kind of shocking for 1966 was that a 17-or-18-yr. old girl would be shaking her booty at the local gin mill. Sure, rich kids messing around and all that, but it was a small town, and she was known to be dating a local working-class lad, and the Blue Whale WAS primarily a bar, unless dear uncle Roger C. was paying for its license.
The Collinses at that point had several setbacks in their fortunes, particularly since Elizabeth parted with Paul, and stopped going out. This isn't an uncommon phenomena in real life--- scions of these former "grand" families" have to slum it with the commoners. We went to PHS with a few of these kids, and some nouveau-riche ones too, whose parents were not eager to send them off to prep school.
L.