setCookie before headers are sent?

Question

I was using this code in my plugin to set a cookie if a condition was met, however after updating to Wordpress 4.0 I got the message:

Warning: Cannot modify header information - headers already sent

How can I set the cookie before headers are sent? This is my code:

    // Add Cookie if Admin
    if (current_user_can('manage_options') || !empty($_COOKIE["loggedAdmin"])) 
        {
        setcookie("loggedAdmin", "true", time()+3600*24*365);
        }
    }