I guess you have to have aging turned off on unplayed sims or use MCCC or something to stop the sages from aging. I kept the original sages until they died and then replaced the sages with the trailer ones from the gallery once, but after that I have just let the game choose them. I don't go to the magic realm that much and I often let my unplayed sims age, so the sages change quite fast even though I play with long lifespan.
All your sages are very inspiring and this thread made me think that I should get creative with the sages once I go back to playing a spellcaster family that often goes to the magic realm :) I downloaded a very cool new magic headquarters from the gallery some time ago, so sages that look more than regular townies would go well with it!