Well, for one thing, drugs are not generally tested on young people. The younger the person is, the less we know about how that young person (college-age, say) would react to modern anti-depressants. (I note with interest that SSRIs--being one of the newest classes of anti-depressant medication--seem to be linked to violence, as opposed to older classes of anti-depressants such as MAOIs [which aren't prescribed often because of nasty side-effects and food-drug interactions] or tricyclics) There just aren't enough studies.

The other thing is that someone who was on anti-depressants to begin with isn't... well... sane (that's a technical term). Due to the chemical imbalance caused by organically-based mental illness, there is likely to be a predisposition to destructive behavior.

Just my 2 cents.