Lyric discussion by RyaNCAA 

Cover art for Old Enough lyrics by Raconteurs, The

I do not think it is so much about them ripping into teenagers.

The first line definitely indicates that the narrator is talking to a female. The narrator could possibly be a mother or father of this woman, one way or the other I highly believe it is a parental figure. Once the parent says "but I detect unhappiness" it indicates that they are concerned for their child.

All the way up to the "what you gunna do now"'s it sounds like the parent is giving the child advice and trying to persuade him or her out of doing something, possibly marriage. Trying to keep their child from making a similar mistake that they themselves once made.

After the daughter makes up their mind the parent then says one of the wisest quotes a parent could tell a child "Think about this carefully You might not get another chance to speak freely" meaning that once you marry this man it doesn't matter how unhappy you are, because when children are involved (when you're old enough) you are not free.