When you are candidate for HT?

I am only 21 years of age and have been experiencing some diffuse thinning/receding hairline. I have been on propecia for about 3 months and things have seemed to subside. Assuming my hairline were to stabilize and not decay over the years, at what age do you think I would be a good candidate for lowering my hairline a little bit.
Thanks for your help I appreciate it :)