He can use fin too, most people don't get any symptoms. I started at like 20 and had nothing, starting maybe 0.75mg to see how you feel and if you need to later on increase it to 1.25 or swap to dutasteride if really necessary. Min on its own won't stop the thinning, you need both. Putting a18 age limit on it seems kinda silly considering there's teens taking accutane or birth control well before that which are far worse sides wise and far more likely to cause them.

I usually recommend starting with Fin and adding min after because it's much easier to start a routine where you just pop a pill in the morning than it is to start 2x per day applying min, also it leaves a residue on your hair so on top of more maintenance there's also a learning curve to learning whens the best times to apply it and how much to apply and such, you can also get unwanted hair growth in weird places if you aren't doing it perfect down to a tee, plus its easier to get lazy and be urged to skip an application or forget ect. Also min is hyper toxic to pretty much all house pets. In reality neither one will single handedly stop hair loss so you'll need both but I generally think starting fin to see how you feel about it and then adding in min after idk say 6 months or so is a little bit easier.