I wouldn't really consider myself old (actually, I don't think anyone over 8 considers 21 to be old.), but I've lived long enough to learn some things the hard way. Some of these lessons could be walked off or at least didn't leave any scars, others were more of a pain both literally and figuratively. To save some of you some time, energy and trips to the emergency room I'll post some of them here from time to time.
Bleach burns, even when diluted with a shitload of water, especially your eyes. It feels like your eyes are on fire, you can't see a damned thing, you curse the man who ever came up with the evil stuff and you wish God would just let you die. I can't stress this enough... Be careful with the bleach.
Thermite burns really, really, really hot and you can't put it out. Don't play with thermite. (I learned this one from a friend.)
Jumping from the railing of a high deck onto a trampoline is a bad idea. It may sound like fun but you'll probably end up bouching right off the trampoline and, if you're really unlucky, slam into the fence located next to it.
When a friend tells you they've decided to learn how to juggle flaming torches and breath fire, be afraid, be very afraid (especially if they're teaching themselves). While they're learning to jugle the torches you may want to consider learning the fastest way to the Emergency room and the number for poison control. When they get really good at it it's super cool, that first while when they're learning is a litte scary though.)