If you don't know the year someone was born or if someone doesn't want their age being stored in Notebird, then follow these steps:
Navigate to the People page
Use the search bar to find the person and click on their name
Scroll down on the left hand side of the profile and click 'Edit profile'
Scroll down on the right hand side and click on 'Birthday'
Type the date in manually rather than using the calendar selector. Use the year 1885 as the birth year. Click Save Person when done.
The birth year (age) will not show in Notebird.
Birthdays are built-in Milestones. So any birthday you add into Notebird will show up in the milestones feed on the Activity page and within your Milestone Reports.