as an adult, i see absolutely no reason why i’m more qualified to buy energy drinks than anyone else.

The reason is cognitive development, especially before the age of 16, the same reason kids aren’t tried as adults in a court of law.

plenty of kids can use energy drinks in a healthy way and plenty of adults can’t.

The main reason people drink energy drinks in the first place is to stay awake longer, which is already unhealthy on its own. Usually this is tied to severe overwork or extreme study sessions, making it even more unhealthy.