I've always thought this question was very interesting, and like most things, I usually arrive at the answer of "it's completely relative to the individual." There are some who marry young and lively happily, some who live unhappily and/or get divorced, some who change out of it or into it. There are some who marry older, and at a varying degree of ages, all with different outcomes. If we want to look at it generally to find some generic, advisable rule, I'd say it would be better for many people to wait until they have experienced more of the "adult" world, although this is not directly related to age. Some people have had to deal with responsibilities and finances from a very early age, and some people almost never have to deal with them. So it's probably more a question of maturity and experience combined.
That being said, I think a lot of people have ingrained notions of what marriage "should" be, and tend to project that onto others. Marriage is a legal contract, on which we tend to place moral and social obligations (I'm including religious under social for convenience). There's no rule that says married people have to behave a certain way, or believe/value certain things. Some married couples might allow sexual relationships with other people, some might not live in the same place, some might be only for money or to connect two families, etc. And so age might have not be a sensible consideration depending on the motivation behind the marriage and the form it takes.
But I think that's mostly me just being pedantic, and the real question relates to those who hold normative ideals for marriage. In that case, the marriage is meant to last and be a kind of lifelong partnership, and so divorce or unhappiness in the household is seen as some kind of failure. And so we'd naturally want to encourage decisions that would lead to happy, lifelong marriages. But that might be putting more value and importance on marriage than there needs to be.
People often do not criticize decisions to enter relationships as harshly as marriages, even though the two are very similar. There are lots of different types of relationships that people relate to things like experience and maturity - the first relationships where holding hands or kissing is a big deal, later relationships where sex is a big deal, relationships where living together is a big deal and then finally marriage holds the position as some ultimate level of a relationship, the most mature and experienced. Like [MENTION=933]Seraphim[/MENTION] said, I think many people want to grow up quickly or be perceived as an adult, and so they want to escalate the kind of relationship they have, and marriage is one way to socially project a statement of maturity.
But I'm not necessarily sure this is a bad thing. People learn from their experiences, and so I can't imagine any sense of wasted time from a marriage that didn't work and ended poorly. The issue then becomes "did marrying young do unnecessary/irreparable/unavoidable damage to the individuals?" In some cases, I'm sure that's probably the case, especially to those involved. But since we can never know the outcome of events that didn't happen, and we tend to always think about good possible outcomes rather than bad (or possibly realistic) ones, it might seem like a failed marriage usually is just bad and harmful. But life isn't easy, it isn't ideal, and it can always be better, or worse. I guess that means we should always strive to make it better, but the problem is often how we might know if what we've done has made it better or not.
My main concern is mostly for children in bad marriages. People can have very difficult and unfortunate lives just because they were born into a bad situation. Thinking about the unfairness of life like that always makes me gloomy.