If you set the birthday question as optional and the user doesn't fill it up, then the variable $user_age is visible on the title/tab of the browser. I guess we'd better to display nothing if the text box is empty.
In the meantime, as you don't need your users birthday info, go to the admin panel, settings - > in the drop down menu "Choose pages" select "users" and in the template for individual user page Chat with {$user_name}, {$user_age} at {$site_name}.
remove $user_age