I am Ars-Magica-gamemaster and want to create some more important NSC for my campaign. Do you have an advice for me, how I may appoint experience for older NSC´s? Should I create a character at the age he will have in the campaign? Or should I create a young character and let him have experience over the years? In the latter case: Do you have a rule of thumb about the Source Quality, the character benefits from each season?
Thanks for your help.
Chiarina.
(Not sure what you mean by "NSC" - not familiar w/ that abbreviation. I'm guessing it's another term for "NPC", non-player character, a SG-run character, as opposed to a PC, Player-character?)
In the end, whatever works best for you and the story. The only time strict accuracy might be important would really be if a Player will take them over later. A good story is far more important than whether the Skill/Arts/Spell totals are perfectly approp for an NPC of that age. (And, really - are the Player Characters going to know? Maybe they look young for their age, maybe they had an amazing Study Source...)
Once built, if the NPC is going to be part of the narrative for saga-years, then you can either fake it (~30 pts/year*) or do something more detailed (season-by-season). Otherwise, create what makes the best story and looks believable, and don't sweat the details. Most of the time, the Player-Characters will never know of half the Abilities and Spells they have anyway.**
(* This is 5th ed. There are full rules for "older characters" on pages 31-33. For 4th ed, creating "older" characters, esp magi, is tougher, and relies on experience with the system, and/or the brute-force, detailed season-by-season method. Source quality would then be approp to your saga, whatever you feel is "typical".)
(* Some SG's, if they are experienced enough w/ creating characters and can think fast on their feet, will completely "fake" NPC's at the table as they go, or run them from the most basic of notes. This is not for everyone, but it's like any theatre - so long as the Players never know the difference, it's just part of the story.)
NSC = Nichtspielercharacter=NPC
You'll find some in Magi of Hermes
Magi of Hermes is an excellent place to start, yes. IN this forum you can find some in the Light of Andorra subforum and you have the excellent "Ranulf from MoH at +75 and +90 " thread by Erik Tyrrell fully developing his own submission to Magi of Hermes to achmagus status. 
Cheer,s
Xavi
Thanks for the kind words. I should warn the original poster that the Ranulf thread relies on advancing a character who has many spells and items that are described only in Magi of Hermes.
Thanks for your answers.
To be true, I had quite precise ideas about the NPC´s. In short, it should be a coven of five or six individual diabolists with some of the possibilities RoP:I is offering. Perhaps the changing of pre-created characters had been possible, but it had been a good deal of work. I decided to create these NPC´s from scratch... a good deal of wok, too, but now I was forced to break in RoP:I. The many possibilities in this book I find to be a little irritating but after one week of my holidays I think I have a chance to play really unique enemies.
Chuchulainshounds hint regarding the rules for older characters on ArM5, p. 31-33 was especially helpful. I only had vague memories about the practicability of this rules. Now I think they are good as they are.
Tharon.