Why is it recommended to wait until a certain age before starting minoxidil for growing a beard?

I see posts of younger guys (late teens, early 20s) posting pictures of their patchy or non-existent beards, asking questions, if they should start, etc. And I oftentimes see it met with downvotes, comments saying "you're too young", "wait until X age to see if it fills out naturally".

But really, what is the risk with starting minoxbearding as soon as you decide that you want to have a full beard? Is there really any problem with a guy in high school or early college, seeing the cool guys getting all the attention from girls, who happen to be masculine looking with full beards before age 20? And then that kid decides that he wants that epic beard life now?

What is the risk or issue with using at a young age, if you can afford it?