I was 20 when I started dating DH and 23 when we got married ... what did I know? (It scares me now to think about it.) But somehow I KNEW, and I still know (did I mention we're going to be celebrating our 30th anniversary in 13 days?). Actually, there were moments of doubt before the ceremony, but none that meant anything afterwards.
But one thing I want to add is that both people in the relationship have to know. Sometimes this doesn't happen, and only one person feels that way, and they end up in a divorce. I'm not trying to be a downer about anything, I've just seen it happen is all.