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 drive a car you own, mandatory insurance, services like uber, and the need not to travel that modern technology presents.
Sex probably includes factors of better sexual education. Higher pregnancy rates are seen in populations with no or bad sexual education. Also, you are 'held to the fire' by the state if you get someone pregnant.
But some of the largest ones are teens aren't getting out in the workforce at younger ages. This means less money to spend on sex, drugs, and rock and roll, along with different social implications for the people you do meet.