Welcome and thanks for posting your query to HCM.
So you are worried about the growth of facial hair (rather beards) in uniform manner. now to solve your queries:
1. Our growth in all manners continues till 25 years of life, so the new growth of facial hair on beard will also be growing till that time. It all depends on the family pattern and genes which we carry from our ancestors. So, basically it will be depending on your parents side.
2. Neither acne or acne scars
and creams prevent the growth of hair. None of the chemicals in acne creams are responsible to prevent hair growth in general. But if the scars are quite deep then at that localized area only you can have damage to hair root, overall is negligible unless you have deep scars all over face.
3. Yes due to exercise we do have bit of increased metabolic rate leading to good hair growth.
Lastly there is no medical treatment in this world which can cause growth of hair in only beard area and your questions are exactly the same which a young man have ( and even i had when I was at your age) so don't worry and be happy.
With best health and beard wishes,
Dr Sanjay K Kanodia