In order to play online games using a PlayStation 3, you have to sign up for a PlayStation Network account. When you sign up for this account you are required to enter your birthday to validate your age. This birthday then resides in your profile. If you forgot which specific birthday you entered into your PSN account, you can find this information by logging into your account and viewing your profile.
Visit the official PlayStation Network website.
Click "Sign In." Type the e-mail address you used to register for your PlayStation Network account and your password into the form on screen. Click "Sign In."
Click the logo in the upper-right corner of your account page that represents your PlayStation Network avatar. Next to the avatar will be the words "Hi," and then your user name. This expands into a drop-down menu. Click "Account Settings."
Click the red "Manage Account" button.
Click "Edit Name and Address." The birth date that you used to register for your PlayStation Network account will be listed on screen.