I’m not overly keen on giving an 18 year old a credit card regardless of whether they’re of legal age, never mind younger.
I got my first credit card when I was 18 and the temptation became too much before I started buying bits of things on it with the thought that I’d get the products right now and pay it off when I get paid. The more I used the card the higher of a limit I kept getting and before I knew it I’d racked up a sizeable amount of debt because it was all a “future me” problem.
Granted all 18 year olds are not the same as me, but I’d be surprised if I was in the minority. It was not a major amount of debt but enough to take a few years to clear, which in hindsight I wish I’d have known more and been more responsible.