Update user profile after form submitted

Question

I'm trying to compute a user profile field using values submitted in a custom user profile edit form. I know that the callback is firing, but it won't set the new value in the field. I've tried every incantation I could think of to set it (right now I'm just trying to flat out set it, never mind the calculation). To debug it, I've tried putting a print_r statement in the callback for both $form and $form_state, but it doesn't print anything.

What's the right syntax? Or is it a bigger problem because it's the user profile, or because I'm mixing a module and a .tpl, or something entirely different? The other form fields are all being set as expected.

This is Drupal 7, btw.

My code, way simplified:

In template.php:

function my_theme_theme() {
    return array(
        'user_profile_form' => array(
        'arguments' => array('form' => NULL),
        'render element' => 'form',
        'template' => 'templates/user-profile-edit'
    ),
  );
}

In my_module.module:

function my_module_form_alter(&$form, &$form_state, $form_id) {     
    if ($form_id == 'user_profile_form') {
    $form['#submit'][] = 'my_module_compute_values';
}   
}
function my_module_compute_values (&$form, &$form_state) {
$form_state['field_age']=300;
}