Managing personal Outlook.com account settings, security, and privacy
Hello,
Since the profile indicates that you are a child account, you cannot change your own birthday on file.
When you go to https://account.live.com, do you see the message "Parent or guardian consent is required for this account"?
If so, select "I'm a parent or guardian" and sign in with a different adult Microsoft account. You can create one if needed. Follow the prompts to give consent.
Once completed, go to https://account.live.com/mykids.aspx and sign in with that adult account.
You should see your original account listed. Select "Edit this child's personal info" to correct the date of birth.
After saving the changes, select "Remove consent for this child's account".
Finally, go to https://account.microsoft.com/family, click the three dots (...) next to your original account, and select "Remove from family group".
These steps are for correcting an incorrect date of birth if you are actually an adult.
Official instructions can be also found here: https://support.microsoft.com/accounts-billing/manage/how-to-change-a-birth-date-on-a-microsoft-account