Gender affirming care is not just for trans people, it should be available to everyone. While I can understand the unknown worries with studies only being done on older men, this is a clear sign that research on younger men should be prioritized.
When I was first prescribed my own hormone therapy as a trans woman, it was made quite clear that there was not a large body of research done on the long term effects. Which was an acceptable risk to improve my life drastically. It's called informed consent, cis men and women deserve the same choice. Just as men do with hair implants or women getting breast implants.
Bodily autonomy should always be the final word. It's their body, so it's their choice.