@Laura5176 I don't use IB myself, never have, never will, but you have requirement options available for IB. Previous recommendations (reviews) from other hosts, verified ID uploaded to Airbnb, and a profile photo. You should check all those. No, you can't use an age restriction to IB.
If you are using Instant Book, you don't have the option to "turn someone down". That's the whole point of IB, that guests can book without host approval.
You do have 3 free cancellations a year with IB, but you don't want to use those up unless you really feel you need to.
If you want to be able to communicate with guests and read any reviews they might have before accepting a booking from them, you need to turn off IB so you just get Inquiries and Requests.
IB is not the best idea, IMO, for new hosts until you get the hang of how everything works.