When you, as an admin or staff member, add a reader to your site, you will be prompted to enter in all required fields, including age. Since a reader's available challenges are based on age, once you populate that field, any available challenges will appear at the bottom of the Add a Reader modal. For the reader to be enrolled, you must check the box next to the challenge(s).
When a staff member is signing up a reader, the staff member is not required to select a challenge. So library administrators, when you train your staff members, remind them to always select a challenge!