I think it depends on how old people are as well. when I was younger I hung out with coworkers all the time - drinks/dinner after work, parties, concerts, trips, etc... as I got older, it happened less and with more select people. it's just a natural transition.
i think it just depends on people.
my first job; that i ended up working at again (quit because i was sick and couldn't afford to work and be in the hospital getting meds at same time) we went to dinner, lunches and movies outside of work.
the second job i've had i ended up shopping once or twice with a coworker that was an old classmate, and hanging out eating and shopping pretty regularly with the stock supervisor (this was at Gap)
the third job i've had i have gone to lunch, dinner as well as movies and even a casino or two with my boss.
all of them were at least ten years if not more, older then i.
fourth job i've met my boss at starbucks, ate lunches, occasional shopping expedition and a LOT of cigarette breaks (i don't smoke but she does) and we were really chummy to the point where other coworkers and managers (she was store) would use that friendship against me. they'd never say it outright but hint that there is favoritism because of that. so sometimes she'd take a lunch first and i'd meet her up later and we'd return to the store separately too.
this pretty much goes on and on. i think currently the job i'm at i don't get along with the boss/supervisor as well as i have with my previous jobs and that somewhat is new to me. i don't really know if i can last long in a job where i don't get along with my supervisors. i don't need to be as friendly level as prior descriptions, i have held other jobs where i am not that tight with the bosses but i always tend to stay longer and ultimately do better when i do because
a) who wants to work at a job where they DON'T get along with their coworkers? maybe if it was a few hours (like ten or thereabouts) a week it wouldn't be so bad. but if you're working close to, basically, or over even--full time status--these people are people you'd see more then almost anyone else; sometimes even SO or family.
i only stay because of the people i work with; not necessarily the work or pay but i am lucky enough to say that because i'm in a metro city where jobs aren't plenty full but they aren't scarce either.
b) if the people you work with, esp supervisors, don't like or get along with you on some level they won't work as hard for you as they should and most likely you'd be stuck in the same position, moving very little in pay raise, and ultimately little to none thanks or recognition for work you do..and isn't that what we all really want? the income and the feeling of job well done.