'The' sexual revolution (i.e. the big one) started in the 60s and pretty much stopped in the early 80s (you can probably guess the reason; it's usually spelled with 3 or 4 letters, depending). That's far enough ago that children born afterwards are old enough to have estranged children of their own. (For an anecdote, which is of course the singular of 'data', I have a close relative born in the late 70s who is already a grandparent).