@Ame0 18 is the age at which guests can open an Airbnb account and book. You can't decline someone based on age if they are 18 unless local laws or HOA regs forbid it.
Make sure to message these guests to ensure that they have fully read your listing description, house rules and cancellation policy and agree to abide by all. Make sure they understand they can't have other people over.
You should also reiterate all the house rules when they arrive. And while you say you're not a chaperone, be aware that many 18 year olds have only recently moved out of the family home or still live there. They may use up utilities as if they are free (think 1 hour hot showers, constant hair-drying, turning up heat or AC and leaving it turned up when they are out, or leaving all the lights on) not clean up after themselves in the kitchen and other shared spaces, expect you to wash their dirty dishes, think it's okay to fire up the washing machine on full load to wash a tee shirt and a pair of undies, etc. In other words, behave like clueless teenagers.
Some young people are super respectful and responsible, but you might need to be prepared to teach them what is and is not allowed and expected in your home.