@Jacqueline1621
Airbnb only requires guests to be 18 or older when creating a profile. Many Hosts of whole home listings do set an age requirement to help prevent parties for larger groups. Some Hosts have HOA restrictions that set a minimum age. Some Hosts mention the listing is not suitable for infants or children for safety reasons.
If you want to set a minimum age for booking, you would need to add that in your additional House Rules on the Listing. Some Hosts also ask for photo IDs to verify age and add that to their addl House Rules. If she is asking what your requirements are, she probably has tried to book another listing and was told she didn't meet the age requirements of the Host to book that property.
If you have concerns about this guest, I would first add any age restriction you have in your additional House Rules first and if you want to add the photo ID requirement. I would also take a look at their reviews. If they are a new guest with no reviews be wary.
You can respond after you have set your age requirement (if any) and simply state we require the primary booking guest to be XX age and that they be on property for the entire stay (be sure to add that to your addl House Rules). This is to prevent 3rd party bookings, where the guest doesn't meet your age requirement, so gets a friend or family member who does to book the stay, even though they won't actually be on property - 3rd party bookings are not permitted by Airbnb.
Airbnb Age Restrictions
https://www.airbnb.com/help/article/1620
3rd Party Bookings
https://www.airbnb.com/help/article/427