Being an old update doesn't make it unsecure, it means that your cb doesn't have all of the memory/hardware needed to update you to the higher versions.

When you start going to websites that start with https:// and you get a popup stating something about your cb not being able to handle secure websites, then that's the point that it's no longer secure.

More than likely the only updates you're getting now are for security, no more added improvements updates.