Filing a support ticket to change your birthday (previously the only way to get this done) is a thing of the past! Now users aged 13 and above can adjust their own birthdays, and parents can adjust the birthdays of their connected children who are younger than 13.
To adjust your birthday, go to your Account Settings by clicking your name in the top right of your Khan Academy account. (Instructions for adjusting child account settings can be found here.)
From there, select Settings. At the top of this page will be the Basics section, where you can click into the box next to Birthdate and select the appropriate date.
Thank you for the requests that inspired this update! Keep the feedback coming!