There are a pile of different answers to these questions (not question), ones that should be asked separately. For example, excluding factors that affect all the questions, putting off drinking is likely a combination of consumer education and the difficulty in getting alcohol pre-21. Fewer adults want to provide their younger peers alcohol because of legal risks. Driving is the combination of increasing expense to d…
I knew a guy older than me who came of age in the hippie era. He had slept with a lot more people than me. He once said "TV channels stopped broadcasting at 9pm. There wasn't anything else to do after 9 o'clock at night besides sex and drugs."
You can now get TV and internet 24/7 and even in small towns you can sometimes find stores and eateries open at all hours. Also, I think historically people turned to sex to get certain emotional needs met that can now be met a variety of ways.
As one flippant example: These days, you can post selfies to get adoration from many internet strangers telling you how gorgeous you are. You don't necessarily need a sex life to get that kind of feedback. Back in the day, an ordinary person who was not a celebrity could not get something like that any way but from a sexual relationship.
In my marriage, we spent thousands of dollars on furniture because it was one of the few things we agreed on and, outside of sex, furniture shopping was about the only time I had his undivided attention. We could have saved scads of money if he had just been willing to have more of a goddamn relationship with his goddamn wife.
For that and other reasons, I have spent a lot of years thinking long and hard about the myriad different reasons people do things. And we just have a lot more options these days for getting some need met without sex that, at one time, we would have had sex to try to meet.
I don't think that's a bad thing.