I think this has been explained so many time before and if someone wants to know about it for information then google makes life easy. Otherwise to pick bones you do not need any thing.
Islam does not believe in digits like 17, 18 as it is different in all parts of the world. In England it is 16, Northern Ireland 17 and in Spain it is 13.
http://www.avert.org/aofconsent.htm
Which one is right?
1. Islam believes in puberty, and that is closer to the nature.
2. Islam believes in
marriage and no hanky phanky, so few people in few countries get their children married in early age as they do not want a pregnant teenager with no sign of the father of the child.
And here are the youngest girls who gave birth to a child,
http://en.wikipedia.org/wiki/List_of_youngest_ birth_mothers