So, I was at my cousins today and he is currently 17 and approaching his 18th birthday, when he dropped a card from his wallet which I immediately recognized as a credit card as it is the same type I have.

Given he is 17, he obviously shouldn't have one as it is a legal issue and both him and the bank could get into trouble so I went about (not telling any of the family because they're the type that think a bee sting would kill you sort of exaggeration) questioning how he got it. Apparently he uploaded his provisional drivers license a few months back and copied & pasted a 0 from elsewhere on the card over his real birthday (so from 94 to 90) and after adding a debit card, still all got approved.

Now he has been responsible with it and never missed a payment so I figure this goes well for him, but what for when he turns 18 as he says he wants to change it to his real D.O.B when he turns 18. Will this affect his credit report and will they pick up on it that he had it before 18 and do anything?

As I said, he hasn't raised any flags (or that is what he has told me) and has repaid everything when it is needed.