I'm glad you changed your mind... but I wanna post my reasons not to anyway, just in case you have any second thoughts
For one thing, sex is a big deal, and the "no strings attached" thing doesn't work so well... It seems like it will, but it's hard not to end up emotionally attached.
Also, your first time should be special, not a random hook-up with some creep haven't even met.
And finally, an 18-year-old who wants to have sex with a 13-year-old is a pedophile, and he would just be using you. You may have thought you wanted to do it, but pedophiles tend to be good at manipulating their victims, and that's really what you'd be. Either that or he's a total loser who can't get any action from girls his own age, which is probably even worse.
Wait a few years, until you're ready to do it with a guy your own age (or at least close - a year or two older is usually fine)... and do it because you want to, not because the adults in your life told you not to. If you want to rebel, dye your hair blue or get your nose pierced, but you don't need to have sex with a pedophile just to prove you're an individual, and you'd only be hurting yourself if you did. Also, my aunt's advice: wait until uni to have sex - it'll be more fun when the guys your age are better at it