The Sony Xperia ion was announced way back at CES in January 2012. It launched in June 2012 running on Android 2.3 Gingerbread with a promise of Ice Cream Sandwich treatment soon after the official launch. Well, the ICS did happen but it took Sony four months to release it. Then came the Jelly Bean promise.
The official Jelly Bean update was scheduled and postponed a few times, but the wait is finally over. The 4.1.2 updates are seeding right now and all the Xperia ion should get the notification soon.
As usual with this kind of updates, the rollout is gradual and it may take a few days before the Jelly Bean ROM reaches your device. You should check both the OTA service and the Sony PC Companion tool.