I'm 21 years old, I started propecia when I was 19, I have been on it 16 months now. I admit it was a big decision at the time. Maybe you should read enjoylifes link to have all the facts about it, side effects with propecia are rare but they can happen but will stop once you stop taking it. Men have been taking propecia for years (for prostate cancer at higher doses) so its not as if doctors know nothing about long term effects of the drug.
When I started to take it, my sex drive really went up for the first 6 weeks then returned to normal. I've experienced no side effects while on it, except my semen can be a little watery now and again. In my opinion you should start taking propecia (or a cheaper generic brand) to save your hair. But remember just because you've lost some hair it doesnt mean you are going to be completely bald, some guys hairloss stabilizes and they dont lose it for decades, your father, older brothers hair can be an a good indication.
Ask as many questions here before you start, it is the best hairloss site on the net.