I am in my 30s and have been on it about a yr, oddly almost exactly a yr. I have some receding up front and a little thinning on back. Noone has ever said anything about the back so it wasnt too bad. It has completely stopped all the loss and it has gotten a little thicker around back. I was prescribed it by a hair transplant surgeon. HE said I was jumping the gun and wanted me to get on the propecia to help maintain the hair that I have. I actually use proscar which comes in a 5mg tablet. It is about 75 percent cheaper and it is the generic for propecia. I cut it into 4 pieces with a pill cutter and it works perfect. The dr told me that it was fine to do it that way.
I think like 2 percent of guys have erection problems but I luckily do not haven any problem there. I am scheduled to see the dr next week to access my progress throughout the last yr. The sooner you get on something, the more hair you can maintain. Good luck.