How to add your birthday and select who can view it on your profile
You can customize your Twitter profile by adding your birthday. You can control who on Twitter can see your birthday on your profile by using your birthday visibility settings.
Sign in to twitter.com or open your iOS or Android app.
Go to your profile and click or tap the Edit profile button.
Click or tap the Birthday section.
Select or change your birth Day, Month, and Year.
You can also review and change your birthday visibility settings. You will see two visibility settings, one to control visibility for your birthday year and the other to control visibility for your birthday month and day. Click a visibility icon (web) or adjust Who sees this (iOS or Android app) to change your settings. Learn more about your profile visibility settings here.
Click Save changes (web) or tap Done and then Save (iOS or Android app).