The studies done by Merc or Propecia were on men 18+ so it should be safe for him at 18.
Dht is responsible for growth mainly between 11-15 years of age in men. At 16 you should have completed the majority of development DHT is responsible for.
Honestly this is a conversation your brother needs to have with his doctor. Assuming he is getting finasteride legally the doctor that prescribed it to him should have explained the risks/benefits.
If he is getting the medication legally, he will be under the supervision of a doctor. In that case i would butt out as it is really your brothers decision.....
I would only say something if he is planning on getting the meds illegally without a prescription. If thats the case he should really talk to a doctor before he makes that decision.
This is not a good place to get medical advice because there are so many conflicting opinions. Someone is going to probably post after me a finasteride horror story... even though most are BS and not supported by scientific evidence. My point is if you want a medical opinion go to a doctor. If you are in the US most Hair loss specialists give free consolations.
From a personal anecdotal perspective, I went bald at 16. Didn't start using finasteride until 18 because i was scared of the stuff i read online. I have never gotten any side effects and i have been on it for over 5 years. However from 16-18 i lost alot of hair, i became a Norwood 3 in those two years. If i could go back in time and tell my younger self to use finasteride i would.
