This page of the Death Stranding guide explains the importance of specifying the date of birth at the start of the campaign. You'll receive a small reward on the day of your birthday.
Date of birth can be chosen immediately after starting a new game. The date of birth will be saved, but the game won't tell you of its importance.
On the day of your birthday, you'll receive an e-mail with birthday wishes from all NPCs managing the communication network. The mail will have an attachment - a small gift containing a pack of likes. They will be automatically added to your "account".
Another surprise awaits you in the private quarters of the hero (you can visit any hideout) - there will be a birthday cake, which you will find on the table.