How to access author data from header action

The global $authordata variable is only available by default when $wp_query->is_author() && isset($wp_query->post) condition is satisfied.

It means that you can’t access $authordata inside a single post page.

You may try to get author data via $wp_query:

add_action('wp_head', function()
{
    global $wp_query;

    $userdata   = get_userdata($wp_query->post->post_author);
    $avatar_url = get_avatar_ur($userdata->user_email);
    ...
}, 10, 0);