I think the sentence
the difference between the behaviour of young people today and that of those in the past
is correct, as that (relative pronoun) replaces behaviour and those (demonstrative pronoun) refers to young people.
Can we also say:
the difference between the behaviour of young people today and the one of yesterday (or and the one of youth in the past)
Can you help me explain why the first one is correct. I can't quite construct the argument, but I know the first example is correct, whilst I'm not sure the second example (using one instead of that to refer to the behaviour) is correct in this case.