What I was saying above is when we adapt the plugin to bp profile search, we only make one change.
Bp Profile Search looks for a datebox field type to use as a date field. Our birthdate field returns "birthdate" and not "datebox" when asking for the type of the field. The only thing we change is return "datebox" as the type of the field and then BP Profile Search could use our birthdate field.
So, if you have Birthdate selected in profile field type, it should work. With the same versions of plugins, it works in my wordpress.
If you can, you could show us what type of field is returning. To do this, you need to modify bps-functions.php, file of bp-profile-search. Put this code in bold, after line 21:
while (bp_profile_fields ()) :
switch (bp_get_the_profile_field_type ())
Then if you reload the profile search page in administration, you can ser the type of each field above. Tell us what type of field it returns. It must return datebox for your birthdate field, if not, there is another plugin or filter interfering with xprofile plugin.