This option is part of your personal profile that you fill out either when you create your account or your edit you personal profile from the account menu after logging in.
We do this as there are a number or forms that use the same information and these forms refer back to you users information in order for continuity between forms and to same you a little time when filling out forms.
To edit you personal data, 1. login to the website 2. from the Account menu, select 'Edit Profile' 3. In t he 'Fields' Section, fill out any data you need. this is where you born date field is located.
I will add a note to this effect on the form though. Thanks for the heads up