Here's the catch. Many Virtues and Flaws have a clear mechanical effect in "normal" play, but no way to account for that effect when creating an older character using "standard" advancement rules (that many xp per year etc.). Twilight Prone, Learn from Mistakes, and Book Learner are just a few examples. By a strict reading of the rules, these Virtues and Flaws should be disregarded until the character starts actual play: a character who's Twilight prone will not start with more Warping points, just as a character who can Learn from Mistakes or is a Book Learner will not start with more xp. Perhaps the Twilight Prone character, aware of the risk, was correspondingly more careful?
Obviously, this means that the corresponding Virtues are comparatively less useful for older characters, and the corresponding Flaws less limiting. But not all Virtues and Flaws in Ars Magica have the same impact on the same time-horizon (think Affinity vs. Puissant), so it's really a much broader issue that, from my experience, is ok to wandwave.