...is a feature rich birthday reminder module that works with Community Builder. Displays a list of upcoming birthdays with the ages of the people. Listed format is fully customizable, and names or usernames can be linked to profiles or private messaging components. Dates can be server, website or GMT.

Just add a CB custom field of type date called "cb_birthday", get your members to fill in their birthdays, and the module does the rest.

Go get it from the CB Addons section in the downloads.