Not a direct answer, but have you found the org-contacts package?

 https://julien.danjou.info/projects/emacs-packages#org-contacts

It supports the BIRTHDAY property. See the note at bottom of the link above
about how it integrates with the agenda.

Example:

** Dad
:PROPERTIES:
:BIRTHDAY: 1955-01-21
:END:



On Mon, May 12, 2014 at 4:42 PM, Josef Wolf <jw@raven.inka.de> wrote:

> Hello,
>
> I am trying to use org-mode for birthdays. I have tried those two entries:
>
> * Calendar
> ** Birthdays
> *** Somebody
>     SCHEDULED: <1970-05-20 Mo +1y>
> *** Somebody1
>     SCHEDULED:
>  %%(org-anniversary 1970  5 20) Somebody1 is %d years old
>
>
> The first entry is shown in the agenda for the current day as:
>
>
>   Calendar:   Sched.358x:  Somebody
>
> I find this a bit strange. I want a reminder a couple of days before the
> event. But I don't want an everday reminder of how days have gone past the
> last event.
>
> The second entry doesn't appear at all in the agenda.
>
> Any ideas what I am missing here?
>
> --
> Josef Wolf
> jw@raven.inka.de
>
>