Unless his parents give him permission or he becomes emancipated, then the age to move out is 18. Sorry.
Although I have answered the question to try to help you, you should consult with a lawyer in your area in person on the matter. In addition, my answer does not establish an attorney-client relationship between us.
No, legally he can't make that decision for himself until the day he turns 18. He's an adult for criminal purposes when he turns 17, and maybe that's where some of that confusion comes from, I don't know. Yes, in reality lots of kids DO move out when they're 17, and nobody really seems to much care or be interested in doing anything about it, but then, that "nobody" factor typically includes their parents. If this boy (your "cousin-in-law"?) has a situation where his parents are abusive, they may not care, and may actually be thankful to get rid of him, in which case there's not a problem there. Technically, it would be a good idea to apply foe guardianship if you're going to do that, because otherwise there might be situations involving his school or medical care that could be problematic otherwise. Often people don't bother with this, and sometimes it works out just fine, but if he were to say end up in the hospital and you needed to be able to talk to his doctor and make medical decisions for him in a time sensitive situation, it could be a mess, especially if the parents weren't cooperative.
If the parents won't agree though, you could be exposing yourself to criminal liability by letting him live with you, and you don't want to put yourself in that situation. The offense you'd be charged with is "Harboring a Runaway", and it does apply to any child under 18. There's a provision in it that basically protects grandparents in your husband's situation, but it would not extend to first cousins. Yeah, it's fairly unlikely that it would actually reach the point of you getting convicted fir it under these circumstances, and I doubt the police would have any real interest in arresting you, but if the parents pushed hard enough (or you just got unlucky and got a cop who insists upon "the letter of te law"), you could actually go to jail for this. It might not seem like a particularly fair solution to this, but perhaps the parents would be more inclined to say okay if you offered to cover all his expenses? If you can afford to do so (or if he can get a part-time job to help out), yiu never know--they might jump at the opportunity to relieve themselves of their parental obligations a bit early..
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