If I can't trust a 20 year old to buy alcohol...

I think that you can trust him to buy alcohol. You just cannot trust the government to refrain from arresting him.

And, to answer the OP more fully, if you can trust a 20 year old to break the law, you tell us what you can trust him/her to do.

Like Vinyl Turnip, now I no longer trust that the OP is competent enough to run a department or manage people.

As others have pointed out, “trustworthiness” has nothing to do with meeting the minimum legal age for purchasing alchohol. And meeting the requirements for purchasing alchohol I’m sure has little to do with the actual job of working as an intern at a non-profit or his ability to perform that job. So I have to wonder why the OP would make such an absurd logical connection.

Being a terrible driver reflects a competency (assuming they meet the minimum legal REQUIREMENT). And being a thief is a decision. These are things that are under a persons control. You can choose to practice a skill to get better at it or not be a criminal. OTOH, your age is not something you can control, nor can you control the laws that are specific to your age group.
As this relates to the OPs competency as a manager and organizer of people - By demonstrating an inability to make a distinction between meeting formal job requirements (things like age, degrees, certifications), competency (the ability to perform tasks to expectations) and personal character (things like trustworthiness which relfect intent and desire), I would be concerned about what other bizzare connections the OP might make or fail to make.
I mean imagine that your boss no longer trusts you to do your job because of some weird arbitrary criteria they invented in their head.

This is incorrect. You can rent everywhere as far I know, although you will be charged a much higher rate.

I blame it on brain parasites.

Dude, that individual is honest and straightforward. And you’re pissed off because he would not do something illegal for you?

Do the tasks you need this person to perform require more or less responsibility than consuming mood-altering substances unsupervised?