Dude, there are credit card companies that explicitly prey on 18 year olds. Just walk through the student union of pretty much any college campus and you'll see their display ads, if not real, live people to help them get signed up.I don’t know if any creditors that are gonna give an 18 year old that high a credit limit.
That said, it didn't have to be his credit card. It could've just as easily been grandma's, or cash that he stole out of grandma's account, or a forged check that he deposited into an account ("look what Meemaw gave me for my 18th birthday!"). When you're on a one-way trip, it pretty much frees you up to do whatever the hell you want, as there will be no extra consequences, and the deed will likely be done before anyone catches onto the stuff you did leading up to it.