The reason why the old think that young love will never last is because of the way they see the young rush into things. To them, when they entered into a relationship, they would take things slow and with a lot of consideration and consultation from both parties. This is the virtue that the current generation does not have.
Because older people understand that true love is all about learning to accept, appreciate & deal with the "total person" - the good stuff & bad. Not just sex, passion & physical attraction. When you're young, esp. in your teens, you're not quite there yet. Close, but you still have a way to go re how life has formed you & your significant other. The extra time allows you to TRUTHFULLY develop yourself & what you have to offer for the long haul. Presenting anything less than your true self, for love & marriage, can lead to constant fighting &/or divorce. Not to mention affecting your children's lives.