For some reason this made me chuckle. You indeed are a minor :D
The problem OP is facing is not that s/he is unable to treat the contributor as adult. Which probably s/he is. The problem is the law does not treat minors as adults. There are reasons for this. Minors are exploited in all sorts of ways. Infact, interns(even adult ones) are treated badly. Once I was working for a company(a world leader) in the UK. A new intern joined. Next day he goes around everyones desk in the team as starts asking if they would like tea or coffee! And everyone was telling them their preference. He came to me as asked this question. This was new to me. I asked him, is this his assigned task? Who assigned him this responsibility to bring tea/coffee to everyone at thier desk? He looked shocked and confused. I said to him that I would be happy he if rather come to me with a work related problem. He did not say a word and went to the company kitchen and bought everyone else their preferred drink! This is how interns are treated. Think what will happen to minors if there was no law.
Disguising work on a startup as "open source contributions" would be fraud. The type of fraud that might lead to violations of child labor laws with hefty fines particularly since the original poster is fully aware of the status of the out of state minor making it a "willful violation".
Also it's not clear to me if a minor can actually contribute to an open source project legally as they have to be able to accept a contract legally I would think.