Hi Josh! It looks like Brian got back to you via email but I wanted to copy his answer here as well!

Unfortunately you will not be able to include the date of birth field on your donation page form. The only fields that can be requested on a donation form are the ones listed on this page: http://nationbuilder.com/donation_variables

Instead, I recommend directing the user to a standard signup page with the date-of-birth field on it either before or after they donate.

This is the code that you would add to a signup page to ask for date-of-birth:

<div class="row-fluid">
<div class="span12">
<label for="signup_born_at_chronic">Date of Birth</label><input class="date" id="signup_born_at_chronic" name="signup[born_at_chronic]" type="date" /></div>
</div>