It's part of a broader spectrum effort by the UN, by their
  UNICEF wing, to improve conditions for children worldwide.
  [1]http://www.unicef.org/crc/ I'm embarrassed at my own country
  (USA) that we have not accepted the United National Declaration
  on the Rights of a Child. We allow working conditions and things
  like underage marriage and many other violations to continue.
  We're accustomed to them. For example, children are allowed to
  work in family restaurants, doing tasks forbidden in many other
  countries. We see it as empowerment but from the UN's eyes, its
  seen as exploitation. The thing is: It probably _is_
  exploitation yet because of how we frame it for ourselves, we
  can't and won't be able to see it. I suspect it is similar in
  the case of child brides. Places where it is traditional and
  existing social customs and laws exist to protect them somewhat
  are nevertheless falling far short of the UN recommendations.
  Poor enforcement of laws are a big reason for it, likely because
  the places where many of these things occur are in areas of
  poverty, where, just like in the USA, things are allowed to
  happen because "poor people don't count". In the US, we disown
  them by saying, "They're welfare mothers having kids to get
  income" and such, dehumanizing them, making them "not our
  problem". Each country commits abuses everyday - one doesn't
  have to look for child brides in far off lands - they likely
  exist in whatever your country is, because few countries enforce
  a strict 18+ only for marriage, as youth liberation movements
  often run counter to child protection movements.

References

  Visible links
  1. http://l.facebook.com/l.php?u=http%3A%2F%2Fwww.unicef.org%2Fcrc%2F&h=aAQF2VU3r