Care has been doing a lot of work on Child Marriage recently. Child Marriage is a huge problem. Yes, there are cultures that consider it not only an option but a way of life and one worth preserving. The problem is that child marriage tends to leave these women economically disempowered, socially restricted, with limited education and access to health facilities and with little control over their lives.
Child brides tend to live in poverty all their lives. Because child marriage tends to take girls out of school, they don't have a chance to develop that important livelihood asset to any person and/or family group - education. They generally aren't part of the formal labour force - both because they do not have education and because it is often seen as culturally inappropriate for a woman to have a career in the poor communities where child marriage is prevalent. Lack of education (or limited educational attainment) is also strongly connected with child illness and deaths. Plus, women who have less formal education are more likely to engage in high-risk sexual behaviour (partly because they are less economically independent), increasing their risk of contracting HIV. Health-wise, young mothers are at increased risk of complications and death in child-birth.
Because young brides tend to marry much older men, and because they are entirely economically dependent on them, there is a dramatic power differential in these relationships, making it very hard to fight gender-based violence.
Child marriages are bad for women and bad for society. Yet, 51 million women below the age of 17 are currently married in developing countries. It's still happening in South Africa in 2010. This is one of the reasons that the updated sexual offenses act in South Africa deals so specifically with statutory rape and similar crimes. Perhaps more discussion about this context would be useful around the now highly-publicised 'Jules High rape case'