My parents' generation, southern-roots had influence, many got married at 16, or in some states, younger.
I've done research, and the big push for the majority to wait until 30s or 40s--including having kids late--was middle-class women entering the workforce en masse after getting degrees, which is the trend now only marriage isn't top of the list for later, any more.
When I was in San Diego the last time around, many neighbors were hooking up, moving in together or not (renting), and then buying a condo or house and not marrying.
It was like a business deal that they could sever whenever they wanted--very practical move, which is how the wealthy used to do it before 'suing each other' became the norm.
That's my observation, and of course there are stats to back it up for those who are into it.