Max Well,
What changed in the 1960s was the more widespread availability of contraception (and then, starting in 1970, legalized abortion) and the lack of stigma attached to out of wedlock births.
I’ve read firsthand accounts of youth in the eighteenth century that aren’t much different from what we see today! Shlomo HaMelech was correct when he wrote that there is nothing new under the sun. Chazal had a point when they made 18 the year for marriage.