OK lets start,
first of all just about everybody's hairline goes back a bit when they are in their teens even girls. This is due to hormonal changes in the body.
secound: The hair on your head is programmed to fall out at different stages in your life due to male pattern baldness. male pattern baldness can come and go in spurts or happen very gradually, it's to do with the way your genetic makeup.
thirdly: male pattern baldness is by far the most common form of hairloss in men(over 95% of cases) and with a near certainty I would say that this is what you have. The most common patterns of male pattern baldness hairloss is on the crown, temples or both at once. I first noticed hairloss on the crown when I was 25, it wasn't noticable , only under close examination of my own eye so I let it go thinking it would be years before it set in properly. In the space of three years my crown has become noticable and over the last few moths my temples are starting to go also.
There's lots of places to buy Finasteride, but I'm not going to push any of them, I'd suggest reading the thread on propceia and then draw your own conclusions.