Hello @Kelsie2,
You can message the guest and ask him to upload a face profile photo. You can also ask him to go through the Airbnb government ID verification process. I have done this in the past for guests who've instant booked without a face profile photo. I got Airbnb involved in a few cases where the guest didn't respond.
As renting a listing on Airbnb is a legal contract Airbnb requires that members are of an age that means they can enter into legally binding contracts. That's 18 in a lot of jurisdictions. If you have doubts it's always best to also ask questions, like purpose of the trip, who's coming with you etc. and judge from the responses or lack of response and the tone of any responses whether you feel comfortable with the prospective guest.
Steve.