Age

I'm probably going to sound like a complete retard but what actually causes people to grow older?

I've never understood what the big problem with experiments on willing human volunteers was supposed to be.

I mean, am I crazy, or is the whole argument ass-backwards? Isn't it better, more "ethical", to use an intelligent, fully informed, willing individual who is making the decision of their own free will than to use animals (which obviously do not fully understand what is going on or the risks involved and furthermore clearly do not have a choice in the matter)?

Sure, there are extremely dangerous experiments where it doesn't make sense to waste human life, or experiments on things where simpler animal biology makes more sense (like genetics, neuroscience, etc), but I don't see the problem with willing human subjects just because they're human.