Why would the "Enable LTE" option be missing in an iPhone 5? I have a iPhone 5 updated with the latest IOS (6.0.1); when going to the cellular options, there is no "Enable LTE" option.

Could this be because my carrier (TIM, Italy) doesn't currently support LTE/4G? But shouldn't at least the option be there?

My carrier will start implementing LTE/4G in the next weeks/months, it should be available shortly in major cities, at least for testing; but if I can't even enable it on my iPhone, how am I going to test it?
The LTE toggle shows up when your carrier settings indicate LTE is available for your device.

If you have data, you can update OTA by going to Settings > General > About and waiting (or tapping on your current carrier settings to see the details).

A tethered check is also available by connecting your iPhone to iTunes and checking for an updated carrier profile for your provider. If there isn't, the most likely explanation is that your provider has not enabled LTE in their carrier settings. If so, contact your provider on whether any LTE (even if not theirs) will work on your iPhone.