You can change the birth date and other personal information you have on file in your Apple account using iTunes 12 link on your computer. The link directs you to sign in to your Apple ID account, where you can change personal information -- including your birth date -- after answering your security questions. Because the same Apple ID is used across multiple services and devices, changing the birthday on your account in iTunes also changes it for any other services and devices that you log in to.
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Launch iTunes and click "Account" in the iTunes Store. If you are not already logged in, enter your Apple ID and password to log in.
Under Apple ID Summary, click "Edit" next to your Apple ID.
Click the link "appleid.apple.com" to access your Apple ID information.
Sign in with your Apple ID and password and click "Sign In."
Click "Password and Security" and answer the security questions that you set up when you created your account. Click "Continue."
Find the section "Select your birth date" at the bottom of the page. Use the drop-down menus to change your birthday and click "Save" when you are finished.