I think SA has an important point; that testosterone goes down with age probably is beneficial to longevity. A friend of mine went on androgel in his 50s after a divorce (yeeehahhhh!) and a few years later was diagnosed with metastatic prostate cancer. He now his lesions in bones and is expected not to live long. Androgens promote the growth and spread of prostate cancers, which ordinarily tend to spread or grow quite slowly in men over 50.
"Where's the REST of me?"
Ronald Reagan, in "Kings Row"