Puberty vs. Learning
There's evidence that puberty can interfere with learning. I suppose that means, in addition to “up-tight” parents who don't want their kids fooling around, yuppie parents who want their kids in Ivy-League schools will also try to raise the age of puberty (once this becomes possible).
After a generation of that, behavior that's commoner among people with earlier puberty may seem low class. We can expect “liberated” behavior to be as disreputable as smoking is today.