>Sorry, but who should buy this?? Maybe 12yr old kids?

I take it you've never seen college boys (or even some young professionals in certain areas) wearing shirts with slogans such as this one (http://i.imgur.com/4vt3m.jpg).

Somewhat related, these (http://en.wikipedia.org/wiki/Truck_nuts) are also quite popular in Texas.

(in case you're located outside the US- American culture, despite its puritan roots and persistent culture, is quite open about crass jokes in that spirit)