I can tell you when it's not appropriate.
When you're 10 yrs old, and being beaten up by some bigger boys, and you think your teacher is watching the whole thing and letting you be pulverised, and you get so mad that when your teacher is finally alerted to the fight (no, he wasn't watching actually), and walks over and says 'so what's the problem?', said 10 yr old replies 'you are, you f___ing bastard!'.
The trials of parenthood...
