Since the launch of the Moto X, Motorola has been pretty good about releasing timely Android updates... though that's been called into question recently. Apparently at least one variant of the original Moto X (the one from 2013) has been lagging a long way behind as well. The Sprint version of the phone only recently got its over-the-air update to Android 5.1 (the one from February). Before now it had been running Android 4.4.4 (the one from 2014). Ouch.
The 5.1 software rollout actually started back on November 5th, but since Sprint says it will be continuing until December 10th, there are probably still quite a few Moto X users who haven't seen the update. In addition to an upgrade to 5.1, the OTA includes the Stagefright security patch. In fact, it's at least somewhat possible that without the necessity of that patch, the phone wouldn't be updated at all - Verizon's 2013 DROIDs, released at around the same time as the first-gen Moto X, are still languishing on Android 4.4.
As always, accepting OTA updates may break root or any other modifications you've made to your phone. It might be best to track down a stock image and revert to it before applying the update, then rooting the 5.1 build. After all, it's not as if Android 6.0 is likely to come any time soon.
- Sprint support
- Allen Robberson