Your terminology is a bit ambiguous, hopefully this clears it up.
Worried as you’re going to lose all your hair in 3 years... highly doubt it. However, similar to almost everyone’s situation, hair loss is something you should get ahead of if you know it is going to bother your everyday life. Many wait until its too severe to start and they wished they would have saved more by starting early.
My advice would be, look at yourself now. If you are perfectly fine with the state of your hair and could afford losing a little more... you don’t need to start any meds - just monitor. If you’re bothered by your hair AND have done your research and chose to take Finasteride... the sooner the better.
And yes, overall hair density falls as we age, Ive heard estimates of 10% per decade.