Plan International have a petition going at the moment to get the Tanzanian government to even up their marriage laws. (Currently boys must be 18 to get married but girls only need to be 14.)
You can sign here, if you're so inclined!
On a somewhat related note, it is deeply weird to me that there are still states in the US where child marriage is legal. And that includes several which, with parental and judge's consent have no minimum age for marriage.
Which...I mean, seriously?