As a 20 year old, you have the luxury of being smart and deliberate about your situation. You may or may not be facing a problem down the road, but either way it is still largely down the road. Find an excellent doc or two and talk to them. Have them help you determine your situation and montitor over time to clarify further. There may be others who disagree, but I would be highly suspicious of any doc who proposes a HT for someone your age, unless the circumstances are highly unusual.
Yes, taking finasteride can help maintain your current density and volume, but beware side effects. Especially at 20years old, there can be side effects you don’t want to have to deal with at this time in your life. At the same time, starting the process of understanding your situation and path forward will help ease your mind.
Best of luck!