What information do you want?
hosts normally don’t receive information about the others guests when a booking is made.
you have the right to ask who’s travelling with her i.e. Family, friends etc
I think you would benefit from reading up more about how Airbnb works - the Airbnb Help website is good for the basics around how Airbnb works.
Anyone over 18 can book an Airbnb you don’t have to be over 25 (unless you are booking a local home for a short stay and have less than three reviews).
Your comment about your 21 year guest booking on behalf of others makes no sense if you believe only over 25s can book.
what information are you asking her to pass on about the other guests what you feel she is reluctant to pass on? You can’t ask for names/ID of other guests if this wasn’t in your house rules when she booked.