I opened my first card at age 18, and I was a few thousand dollars in debt within a year or two. It took me a few years to get out of it, and to really internalize that when people say credit cards are dangerous and can ruin you financially, it's not just preaching or hyperbole, but it applies to every single person, even me.
BH I've learned some more discipline since and can now use a credit card in a positive way, but unless people realize and deal with the risks, I'd say don't jump to get a credit card on your 18th birthday. The "good credit" is not worth the tradeoff IMO.
ETA: Just saw Yehuda57's last post, and he hit the nail on the head.