I would not take propecia at 19 years of age. Right now your body is a ball of hormones and playing with chemistry just to save hair follicles is too big a gamble in my opinion. It isn't just a magic pill that saves hair, it could possibly affect any other bodily development and more importantly your mental health which is still maturing.
Judging by your questions your knowledge of Finasteride seems limited for someone who is starting it so soon. I would take time out to assess what your current condition is and what is your end goal. Once to commit to meds it's a life long journey of commitment and adjustments and everything else.
You've got plenty of time, if you can't wait it out to do more planning than due to aggressive loss than whatever condition your current hair is in is probably not DHT related. Obviously you have not explained your current situation but there could be a multitude of lifestyle factors affecting your hairline.
Trust me, take time, assess your hair and loss pattern, maybe speak to proper medical professionals and above all else on start meds if advised and if you are adamant.
I hope this helps man and it might seem intense but honestly in the world of hair you're just a puppy and there's a lot of factors to consider.