It depends on the reason. In this case, I'm assuming you're talking about selling something. I would phrase it as simply, "You're underage, so I cannot allow it." Or "You're not allowed to purchase this since you're underage". Saying, "..you can't DO it" would be more appropriate when you're talking about someone who is physically incapable of performing an action or completing a task. Using the word "do" is for an action or something similar. You can still say it the way you had mentioned and get your point across but it is not formal. Hope this helps!

Maybe, more appropriate to say: "You're so young, so you can't do it."

yes you can. you could also say xyz is only meant for adults. or the law/rule does not permit minors to do this.