Hmm which study are you looking at?
This one did a 10 year follow up and had maintenance or regrowth in 86% of patients:
"It is interesting to point out that of the 113 patients followed for 10 years, only 14% worsened, whereas the remaining (86%) had benefits (21%) from the treatment duration or (65%) persisted in their improvements."
From another study
https://www.sciencedirect.com/science/article/pii/S0022202X15529357#:~:text=The results of these 5,above baseline over 5 years.
"90% of finasteride-treated men either maintained (no further visible hair loss from baseline), or sustained visible improvement in, scalp coverage over 5 years, while 75% of placebo-treated men sustained visible deterioration in scalp coverage"
I think you're right that age affects response, and also race. For example I believe Asians respond particularly well to finasteride. The 20% or so that continue to lose hair on finasteride may need to take dutasteride instead, though even that may not work. So it is true that finasteride doesn't work for everyone, but if you're part of the 80% that does respond to finasteride, then at the very least it should preserve your hair until hair cloning becomes available.