The number of fully-statted 5th edition NPC magi is actually itself rather small. Most only include things like personality traits. I suppose that's why there was an Open Call for a bookful of fan-generated magi.
I'll take my books on the train this morning and see if I can plot XP vs Age for the magi in GotF and LatL.
Yeah, its true that 5e magi are few on the ground. I was including other editions' magi in my discussion. I own everything except a couple of adventures for 2nd, 3rd, and 5th edition. I have almost everything for 5th also (don't have the normandy tribunal book).
But even with the simple descriptions, they don't give you any reason to suspect any of the Gifted ones significantly lack talent in the way I'm discussing.
That's right - Ars characters are very sensitive to the number and kind of Virtues (and Flaws) which apply to the particular talent they are most powerful in, but Characteristics are usually not so important for magi (because of the Art scale reaching totals in the 40-60 range, and that in 5e Characteristics can't boost learning).
(Having said that - a focussed non-magus with Virtues to back up Abilities and Characteristics can amaze and astound, and become the stuff of legend - but not many are so specialised, even among the myriad mundanes...)
A focussed character (Puissant Art, Affinity with Art + Magical Focus) can be frighteningly powerful from an early stage. Add in Book Learner and an excellent covenant library, and determination to study, and they can race ahead...
That's not the only route to power: lab work + devices + V/F can produce amazing devices;
Mysteries Initiations can stretch a characters V/F at the expense of much time away from normal study;
Hermetic Breakthrough research can take you into completely new areas.
A few NPCs have a similar degree of focus - in a Saga they may be specifically rivals to the PCs - not in their precise field, but "I can do better than him, look at my Amazing Show!", but many NPCs (and not a few PCs) have other drives, or mixed drives:
characters usually want to be fairly powerful in "some" aspect, but to be well connected, and liked, and to take pleasure in the world. They collect things, and help out friends.
It is the monomaniacs who amaze and astound (while we later tut slightly about their lack of other skills...)
Here's the promised analysis. Funny how two and a half-hour in public transportation can help 
I've summed up arts, abilities, spells and enchantments to produce an experience total. I haven't tried to account for the savings or bonuses from virtues, so that if a magus has a "+50xp" virtue, he's just got 50 more XP in the end, same with affinities. I may have forgotten some things here and there, but I should still be about right.
Iasper (LatL p45), Gauntlet at 22, Age 26, 647 XP
Anacrôn (LatL p84), Gauntlet unknown, Age 56, 1892 XP
Tabanus (GotF p21), Gauntlet at 21, Age 38, 1015 XP
Felicia (GotF p23), Gauntlet at 24, Age 24, 690 XP
Philippus (GotF 59), Gauntlet at 21, Age 125, 4582 XP (ouch)
Imanitos (GotF p73), Gauntlet at 22, Age 55, 1686 XP
Falke (GotF p96), Gauntlet at 23, Age 45, 1433 XP
Ioannes (GotF 106), Gauntlet at 23, Age 23, 760 XP
Without trying to account for the pre/post apprenticeship differences, the XP/Age ratio remains pretty close to 30. Things diverge more if you only consider post-apprenticeship differences. Anacrôn, but mosty Philippus have more XP than expected, which can probably be attributed to non-abstracted lab work calculations later in life.
There you go, food for thought;
I am of the opinion that the Gift--at least as it is cultivated in the Magi of the Order of Hermes--is skewed markedly toward more intelligent individuals. There may be a few "savant" Magi with exceptional Gifts and otherwise unremarkable intellects, but I believe that Hermetic Magic tends to follow the patterns seen in science more than the arts. That is to say, more intelligent minds grasp the concepts faster and tend to push them further.
Of all the Houses, Bonisagus is the one which probably has the highest "average" Intelligence because of their intensive Apprentice selection. They favour the most brilliant, or at least those with the greatest aptitude in a given area of magic.
The 5th Ed. depiction of House Jerbiton differs from my longstanding treatments in a most ironic fashion. I've long thought of them as being among the foremost scholars of the Order, emphasising their links to Classical learning and therefore the ongoing natural philosophic exploration of the age. Far from being inclined to leave off the study of magic, they use it to enhance their studies of the world.
Yet consider the case of an artistic personality with the Gift. Focusing on a few areas of study, he or she could explore permutations of the arts almost impossible for mundanes. They might not develop faster in terms of raw Hermetic power, but compared to their mundane artistic counterparts they could achieve phenomenal levels.
The ultimate irony is that I don't think that there is such a thing as a "weak" mage--only one who is less puissant than his or her contemporaries. Someone who dabbles in numerous minor magicks can still perform feats outside those of mundane ken, and it takes very little to turn those feats into success. Much of it is a matter of resourcefulness. A good look at the history of the "real world" demonstrates just how much humans can achieve with comparatively limited tools. Grant them access to the possibilities of magic, and there's an almost exponential progression.
In fact the game does alllow characters to get a non-standard loading of characteristics. That's what the Improved Characteristics Virtue and Weak Characteristics Flaw do.
Also, it is possible for magi to invent rituals to increase both mental and physical characteristics. So, I wouldn't personally have a problem if a character who had invented/learned such a ritual began play as if he had already cast that ritual on himself. As long as it was plausible in the character background for him to have access to the required resources (lab+vis).
It is also possible for a character to have worse characteristics than standard due to accumulation of aging points.
Well, that's true but its an entirely different issue.
Omly if you define the virtues and flaws as being outside the purchase system. I don't, because they clearly have a currency (flaw slots).
A character who has lost points due to aging -is- standard. Voramein's point, as I understand it, si that NPCs shouldn't necessarily use this system at all, and so there's no need to look for mechanical explanations for their better or worse stats. They just have them, ab initio.