Thanks Trevor, always a pleasure 🙂
For folks with a similar issue, what I learned is that the “include” function specifies which taxonomy terms will be added to a select list, but it does NOT pre-select any term, so the default is still “all categories.” Trevor provided a jQuery snippet which I was able to modify, which sets the select option of the age field to the value I want on that page. My version is roughly:

<script>
jQuery( document ).ready(function(){
 var age = 'senior';
  jQuery('select[name="_sft_ages[]"').val(age);
});
</script>

And I just modify the age variable depending on which age group’s page flow I’m on. So far it’s returning the results I want, and I can see the correct age group passed in the URL.
Thanks again Trevor!
–Carina