If you could only develop schizophrenia young, it wouldn't make much sense to call it "age-unrelated". You can get chicken pox when you're old, too, and in fact it's particularly deadly to the elderly, but there's not much place in aging amelioration for anti-chicken-pox research. It's best addressed elsewhere (also, yes, basically solved already, but I'm trying to make a different point).

23.5% of schizophrenia sufferers is a minuscule quantity. What percentage of people over 40 develop schizophrenia? Over 60? What percentage of people over 60 develop one of "cancer or heart disease"?

> a healthy body and a decayed mind is in some ways little different than a dead body.

I don't disagree at all.

> finding real treatments for mental illness is probably the most important part of longevity research

No, the most important part of longevity research is getting the body to last. A dead body with a "healthy mind" (not clear what that would mean) doesn't qualify as longevous by any standard. Someone who won't go senile for 150 years is pointless if he dies at 82 like a normal person, and while that scenario is admittedly a kind of progress it (a) is undetectable, which makes research difficult, and (b) has no application at all until we can get the guy to live longer. Longer lifespans are more fundamental, are immediately visible, and have immediate application; they can take advantage of natural variation in the onset of mental failure.