All of EA TC's mages are sacred.
Nothing "special" happens at Dom10, including old age being removed from sacred mages.
Dom10 happens all the time, in face I even start at Dom10 in some builds, and old age is still old age.
You said he's fairly new to the game, that's fair enough. Most people are prone to making somewhat bizarre observations early on in learning a game this complex. I know I thought some strange things for awhile myself.
