In addition to the calendars you create, there is a special calendar called Birthdays. You can toggle this on and off in the calendar view menu in the toolbar.
The Birthdays calendar is automatically built from your address book. It will show the birthdays of every person for whom you have specified at least a day and a month of birth. For contacts with a specified year of birth, the events will say how old they will be turning on each birthday.
The birthdays calendar is only available in the web interface; it is not available over CalDAV. Most calendar apps can build their own birthday calendar from your address book.