As with most things, I'm with Jackie on that.
Even for adults, if you think you're in love (or are, regardless of the outcome later on), no amount of naysaying is going to change that unless it's like "Uhh dude here is a picture of her sucking a guy's dick for a baggie of cocaine" (I like to imagine people talking about hardcore drugs and then saying "baggie"). Granted, if you're already waxing and waning on the relationship or something is already wrong, that's a different story. But yeah most of that horribly pretentious, nose-in-the-air snubbing of "Oh I knew she/he was bad news" is such bullshit. It's one of those things that feels like people say that shit to make themselves feel better for whatever the reason. People love to judge, that's about as close to an infallible "fact" as you can get.
Also like what Cram said, people clearly know jack fucking shit about the relationships of others and how things really are underneath the surface. Fact of the matter is they still love to judge and pretend they know their ass from a hole in the ground when the reality is that sometimes even the people within the relationship don't know that it's going to hell. I've had it go both ways. People tell me that we're perfect and should/will get married in the future and it goes to hell. Also that we're destined to fail and that we're so wrong for each other and even though we "broke up", it didn't go to hell and she's the only ex that I'm still very good friends with. Also we fucked the other week so. THAT'LL SHOW YOU, NAYSAYERS.