From the mail screen, click on your initials in the circle in the upper right corner, then click the “My Profile” link. In the upper right corner of the “Profile info” panel, click “Edit profile info” and you can change the date of birth.

But here's what Google support says: “When you sign up for a Google Account, you may be asked to add your birthday. Knowing your birthday helps us use age-appropriate settings for your account. For example, minors may see a warning when we think they've found a site they may not want to see.”

Google celebrates events with Doodles. Users can find their birthday doodles on the Google Doodle website by entering their date of birth. The website curates all the doodles created over the years and allows users to search for doodles and events that happened on their birthday. Google celebrates events with Doodles.