Baldness comes from hormonal changes as you age. Testosterone becomes metabolized into dihydrotestosterone, which for some reason thickens the skin on the scalp, cutting off nutrients to hair and causing it to die and fall out (aka, baldness).
For some reason, this is somehow related to prostate problems. In researching prostate problems, drug companies ID'd a side effect of one of their drugs, finastriside, that stopped this metabolization of testosterone into DHT, which causes baldness.
I'm balding, have noticed it since my mid 20's with lots of hair on my pillow, in the drain, and in the mirror. I use finastriside (Propecia/Proscar) and have a lot of hair, and my control (twin brother) doesn't, and he looks awful. Finastriside stops hair loss in its tracks, and takes effect in a week. It's pretty amazing. I've never noticed any sexual side effects, as a lot of people have cited. But taking too much does make your nuts ache in an unpleasant way.
Guys on this board always say "shave it" or "accept it". Fuck that. You dont have to be bald if your dont want to.