What do you think about my plan to anonymously turn in an underage drinker based purely on a rumor?
So you didn't see them drink?
No, but a reliable source said that she drinks and that is enough to get her in trouble.
It is good to report people when they are doing things they are not supposed to but it is not okay when you don't know for sure. For all you know the person made it up and you are getting someone in trouble for nothing.
No, they wouldn't make this up.
I know you trust the source who told you this, but accusing someone of a crime based purely on hearsay is still wrong. Since you didn't see them do it, it's best to just leave it alone.