Quote Originally Posted by Ludens
Basically you get them after a set date, perhaps 107 BC (IIRC an EB member stated that as yet all reforms are coupled to the Roman ones), but if one of your generals is defeated in a battle with Kataphraktoi being present on the opposing side (Phalava I assume; I don't think there are triggers for Sauromataen or Bactrian Kataphraktoi) he can get a trait ("Intrigued by Kataphraktoi") that may lead him to develop them earlier. The EB traiters have not yet revealed how this trait works.

Also, don't expect them to replace Heitaroi. Hellenistic Kataphraktoi lack both the maces and the elan of the Phalava elite and were always held second to the Heitaroi.
Oh thanks for the info.