This is always a hot topic no matter what...But I would definately say you need to look at exactly why the person in question would want to do such a thing....I know it was already mentioned; like say someone is extreamely sick with an uncurable disease, they really don't have much time left anyway, and the natural course would would what time is left would be spent in a lot of pain, not really be able to do much, exc. then I would say let them go ahead if that is there choice.
BUT some teen that is simply depressed because a boyfriend dumped them - No way...
So it's not a clear cut thing, it's why, and a lot of other factors.