I think a lot depends on the people involved... thier personalities, their circumstances etc. If you've found 'the one' at an early age, then be thankful you've found him or her so soon! However, I'd be hesitant to say an early age of marriage is indicative of maturity. I too was called 'old for my age' (and in retrospect, yes, I was), but that can make it more difficult to meet someone who shares similar values, interests, depth of thought etc. In fact, I'm still looking. This is true of many who marry later in life. We're waiting 'till we find the right one, be it sooner or later. Unless you've really found him or her, and the time is right, it's too soon.
That said, I think there's a lot of wisdom in what mlukas wrote. I've seen many bright, otherwise mature people who thought they knew what they wanted in life, change radically as they've lived through their twenties. Relationships can suffer for that, as one or both people grow apart. If one marries too early, the risk of growing apart is increased. Sure, tying the knot early can work, and God has blessed many such marriages, but they're generally running against the odds.
Invisiblebabe: Increased self-awareness doesn't automatically mean one will, or should, marry earlier. Many introspective types actually remain alone for a long time, for reasons similar to what you describe. But I do agree that it depends on the individuals in question.
