In my view, (and I have tos ay it makes sense, at least to me) when a magician of dessi attains level 20+, he stars learning the greater arts, and by the time he gets to level 26+ he is probably a elder magii.
The greater elder arts are detailed in magic of magnamund; if you want to genrate elders for your game, I would suggest after level 20, you follow the normal dessi progression from level 1-20 (again) but everytime they gain an elder art, they ought to gain a greater elder art. Arcane protection keeps increasing, and they probably get the shianti's arcane resistance feature as well. Plus, they probably also get the features of the Aertborn table, as they learn new arts.