can we use date_select as one observe_field.

hi all,
i want to find out the age of a person in the simple registration form
without submitting the form.
we can use link_to_remote or submit_to_remote(....), but i don't want
to use any link or button to link to the controller. Can we use
date_select as one observe_field or any other possibility to do this..

plz guide me to do this...