@Karen2636 No. This is a third party booking, which are against the Airbnb Terms of Service. If this was an Instant Book, tell the booker that the 18 year old would have to set up their own account and book for themselves and that you'll need to cancel the reservation as it stands. Then cancel it under your 3 free IB cancellations, stating that you are uncomfortable with the reservation. You don't even know for sure if it's the parent booking- it could just be some other 18 year old who has an account.
I'm a little confused- you say they have booked, but you ask if you should accept it. The only way someone can "book" is with Instant Book. Otherwise, it is Request to Book, which isn't a booking until you accept it. So if it's a request to book, just tell them what I stated above and ask them to withdraw the request. if the don't withdraw it within the 24 hours you have to decide, decline it.
Be aware that booking from people of this age often turn out to be someone looking for a place to throw a party, especially if it's only for 1 night- new hosts are often targeted, as they know you are inexperienced. If I were you, I would turn off Instant Book, if that indeed is what you are using, until you become more experienced with hosting and vetting guests, so guests have to communicate with you and you have to approve the booking.
