The field should only accept values from 39 until 47.
It can be empty.
It should be editable on the admin member edit page.
It should be visible on the public user profiles.
This hint is about the second bullet: We need to add a field to the profile.
It's important that you know that the profile is a model. Don't know what those are? Take a look at the Django documentation (don't forget to select the right Django version in the bottom right corner) to find out more.
Now that you know what a model is, you should also know what a field is. Note that there are multiple types of fields. Select the right one and add it to the right model.