DUI in the 1970s was just not as much a worry. The BAC limits were higher and enforcement was lax. Driving after a few drinks, as long as you weren't totally blotto, was pretty common.

There's probably some mathematical relationship between car dependency, time, and urban sprawl too.

Back in the 70s there may have been 1 or 2 affordable neighborhoods in your metro area where people in their late 20s could buy a house -- and they were both probably fairly close to the city center. Nowdays, I'd say those neighborhoods are likely to be farther away from the city center (exurbs) and thus farther away from each other.

I think about this with my hometown friends in the PNW. Growing up we all lived in Bellevue, Redmond, and Kirkland, and now those areas are all too expensive and folks are living in Issaquah, Duvall, Renton, etc.

Live in NYC and my wife and I were just commenting on how when we learn about our friends' lives in the suburbs in our home states they really seem to lack a social life. It's like it just dies in your 30s. This seems to be independent of obvious things like having kids, as the parents I know who live here seem to have similarly rich socialize lives to the DINKs I know.