@Serena388 Airbnb does not provide any built-in means to set a minimum age requirement or for you to see guests' IDs. In fact Airbnb's terms of service prohibit asking guests to send you their ID prior to check-in, unless you are legally required to do so, which is not clear from your post.
What you can do is enter the requirements in your house rules and your listing text, and enforce them yourself at booking. In your situation I would not use instant-booking.