My hair started falling out at age 18. I wasn't one of those unlucky guys who ended up like Captain Picard by 20, but it was definitely receding. At age 21, I started taking finasteride each day (I would break a Proscar tablet into four pieces with a knife and take one piece each morning). It definitely slowed my hair loss down to a crawl and might have even reversed it a little on my crown.
I have been taking the pills almost each day for the last five years now. Yes, my hair has still continued to fall out, but very slowly. My most dramatic hair loss was between ages 18-21, which was before the pill.
Currently, I am a Norwood 2. My crown is kind of thin, but the most pronounced part of my hair loss is in the front hairline, which has an M shape. Think Jude Law but only half as bad.