This has happened to many classmates of mine and teachers/parents may not know how to fix it, which is frustrating when you're trying to study for the SAT and just want to link your data. Here are some ways you can fix your birthdate and get back to studying. Note that these will need to be done on the website for the most part.
Fixing it by yourself:
This may not work if your account is under a Parent Account, but if it isn't, this will let you fix it yourself. Open a Request Form, select "I need to adjust a birthday" in the dropdown menu, fill out the required information, and submit the form. The Khan Academy Team will contact you via email when the issue is fixed.
Fixing it through a Parent Account:
If your account is under a Parent Account, your parent can fix it through their account. Have them click their name at the top right of their screen, choose Settings, choose Settings for your account, and directly edit your birthdate.
Fixing it through a Teacher Account:
If you use your account for school and are registered in a class on the site, your teacher can also fix it through their account. Have them click their name at the top right of their screen, choose Teacher Dashboard, choose your class, go to Students, click on your account, click Settings, and directly edit your birthdate.