Of course its lawful for anyone over 18 to book why would you think otherwise @Andrea8411
And it's absolutely nothing to do with you whether his parents approve of his relationship with his girlfriend or not. We are accommodation providers not the moral police !!!
At 18 you can get married, fight for your country, buy a house, sign a contract. And of course sign up to book accommodation on the Airbnb platform.
of course we should vet guests to ensure they are a good fit for our listing but I don't discriminate based on age.
a few weeks ago I had a lovely couple who were 19 they were one of my nicest guests. Lovely manners , looked after my home well and a delight to chat to .