Why A Girl This Young Is Giving Out To A Man In Marriage?

With the way girls who got married as early teenagers are suffering, this should just stop. A Nigerian singer Duncan Mighty shared this photos via his page and wrote:
"Please my people, lend your voices to End child marriages. Many child brides develop fistula as a result of early pregnancy that subsequently will either kill them or render them disabled for life. 
This little child needs a PENCIL and not a joystick... #IstandagainstChildMarriage"