26.
The problem is, by the time you notice the sides, you might well be already fucked. It's not simply a case of jump off the treatment and you'll be fine within a week or two. The average side effects lasted 3 and a half frickin years in the study I linked above, and some sides were permanent. Some patients only took the drug for a few weeks. And you can bet the number of patients with ED was under reported as it's an embarrassing issue for most men.
I'd broadly agree hair loss matters more when younger than older, although I'd set the 'acceptable' age at 35, ideally 40. But if we're agreed on that, then you must surely agree with my position. You do 1 hair transplant now and use minoxidil to get say, 5-8 years of decent hair. Then you go back for another hair transplant to take care of the areas that have thinned out since then, to buy yourself another 5 years or so. If after this you have no more donor left, well by the time it becomes an issue you'll be much older and you can shave it the f*** off with less social stigma and without hampering your dating prospects as much. There might also be a cure in that time, or at least a better treatment alternative than finasteride.
I'm only 18. Transplants are not an option for me. ''They found 1.4 percent of them developed persistent erectile dysfunction''. Aka 1/100. I think I have better chances buying a ticket lottery. Therefore the 1.4% is not anything scary for me.
Edit: I cannot afford a hairtransplant anyway. On the other hand, Propecia only costs me like 50$/year.