That's an over simplification. Mavericks is a free upgrade from Snow Leopard onwards (That's the previous to the previous version of OS X BTW). If you don't have a licence for Snow Leopard, then not only you part of a really tiny percentage of users) but buying Snow Leopard is only £14. It's nothing to do with the age of the system, it's because Apple only offer Mavericks through the App Store and OS versions prior to Snow Leopard don't have the app store. So worst case scenario it costs £14 to upgrade your Mac(s). But seriously, if you've survived this long on Leopard then just stick with it and keep Windows ME on your PC while you're at it!