I think it really started to change fast right about the time I left college, but definitely at that time the cultural norm was that the fun ends after college*, that it was time to get serious, settle down, get married, start cranking out the kids, etc. I don't think this was a good or bad state of affairs - though I think the cultural and peer pressure about it was bad - but nor do I think having a bit of time to bumble through your twenties is a bad state of affairs.
*Yes I know not everyone goes to college, was talking about the view from a certain socioeconomic perspective.
