Pretty hard to give a good answer to your question. I recognize myself in your younger brother and you in my older brother. My older brother started going earlier than me (obv) but he never told me anything about what to expect regarding baldness or what treatments there are out there. I don't blame him because I was too young to get on any treatment anyway when I started balding and on top of that, my brother is pretty introvert so he doesn't like to talk about these kind of problems, not even to people that are close to him.
Anyway, in the end I would have an honest conversation about it with him in the end. It just sucks he can't take finasteride right now because of his age, but it's good for him to know what he can expect. There are too many balding people living in denial ("Oh it must be Telogen Effluvium, it will grow back!" or something like that) and that can break him up in the end. Besides, you know what it's like to have male pattern baldness, you can relate and you're his bro. Therefore, if there's anyone who can have an honest and deep talk about this with him, it's you.
On the flip side, for some, ignorance is bliss. It really comes down to what type of person your brother is. Me personally, I would've wanted somebody to guide me through it.
Anyway, in the end I would have an honest conversation about it with him in the end. It just sucks he can't take finasteride right now because of his age, but it's good for him to know what he can expect. There are too many balding people living in denial ("Oh it must be Telogen Effluvium, it will grow back!" or something like that) and that can break him up in the end. Besides, you know what it's like to have male pattern baldness, you can relate and you're his bro. Therefore, if there's anyone who can have an honest and deep talk about this with him, it's you.
On the flip side, for some, ignorance is bliss. It really comes down to what type of person your brother is. Me personally, I would've wanted somebody to guide me through it.