Create new custom field that calculates age

In order to be able to query by age, you will need to create another custom hidden field to store the date.

You can then hook into the acf/save_post hook with a priority higher than 10 in order to update the age field value whenever the field is created/updated.

Here is a gist of the code that should work

<?php

function my_acf_save_post($post_id)
{

    // check if post type is persons
    $post_type = get_post_type($post_id);
    if ($post_type != 'people') {
        //return if its not a people post type
        return;
    }
    // get year of birth value
    $year_of_birth = get_field('date_of_birth', $modelID);
    //calculate the value of age
    $age = intval(date('Y', time() - strtotime($year_of_birth))) - 1970;

    // update the age field
    update_field('age_field', $age);

}

// run after ACF saves the $_POST['acf'] data
add_action('acf/save_post', 'my_acf_save_post', 20);

?>