A good resource for building magi at various power levels is the Magi of Hermes book from Atlas Games.
It presents fifteen Magus characters, with backgrounds and story hooks, but also fully statted at several points in their life: right after their Gauntlet , 15 years later, 30, 60 or 75. It's both a good source of powerful NPCs and a good template for building them.
If you want magi even older than that (75 years active + 15 years of apprenticeship + ~8 years of childhood = ~100 years old), it shouldn't be a problem to check the differences in stats between, say, the +60 years and +75 years versions of the same character to see how much of a bump you should give. Keep in mind, of course, that it gets incredibly hard to raise scores at those ranges, so you should check out the differences between +30 to +45, +45 to +60 and +60 to +75 to see the average rate of slowdown, as well. Or just approximate and give a bump to the stats.