I read an entry on Timothy Ferguson's blog about how to re-model Ars Magica 5th Edition Longevity Rituals for more interesting flavor and to shift the apparent aging of Hermetic magi. Because AM5 lacks a core central mechanic for adding and removing Virtues and Flaws post-character creation, Warping and Mystery Cults effectively became the de facto ways for doing it in supplemental books. While I don't think Mr. Ferguson's idea of longevity ritual as a Hermetic Exoteric Mystery is a bad one (especially for flavor, which the RAW longevity ritual rules conspicuously lack), it doesn't address how Hermetic magi age. I.e., it does not fundamentally alter how the Aging tables work, nor definitively when magi will start adopting Longevity Rituals. That is, arguably, the larger problem with the RAW: given good enough rituals, many magi will look like they are in their 20s-30s well into old age.
Mechanically, I believe the simplest way to address this is by adding a new "apparent aging" range to the bottom end of the Aging table. The precedent for this can be found in the core Aging table itself. Aging Crises are a big deal in AM5. Even if you resolve any given Crisis, you still acquire Decrepitude and potentially a bunch of Aging Points. To make Aging Crises a potent threat even at relatively young ages, both 13 and 22+ on the standard table result in "sufficient Aging Points... to reach the next level in Decrepitude, and Crisis". Since most characters start rolling in their mid-30s, that means that, barring modifiers, a roll of 9 (+4) would immediately result in Decrepitude and Crisis. 13 remains a real threat for most characters unless they have a truly solid Longevity Ritual. It's a landmine on the Aging chart relative to the results around it, but its presence is important because it significantly increases the probability of hitting Crises at young ages.
Similarly, putting an additional apparent aging range low on the Aging table may seem unintuitive, but it can produce the mechanical results you want. By overlapping the main apparent aging range (3+) with a negative range, you can create a higher probability that magi will slowly continue to gain apparent age even with powerful Longevity Rituals.
E.g., we could add an apparent aging increase result between -5 and -7. Let's say a maga has a total of -7 to her Aging roll due to Longevity Ritual (-10), being a maga in a Spring (-1) covenant, and her real age (37, +4). The die rolls produce these results:
10-7 = 3, apparent age increases by one year
9-7 = 2, no apparent aging
8-7 = 1, no apparent aging
7-7 = 0, no apparent aging
6-7 = -1, no apparent aging
5-7 = -2, no apparent aging
4-7 = -3, no apparent aging
3-7 = -4, no apparent aging
2-7 = -5, apparent age increases by one year
1*-7 = on subsequent die, 5+ = apparent age increases by one year (and other deleterious effects on a 9 or 10), 2-4 = no apparent aging, 1 = roll again and quadruple
As another example, with -12 to the total roll, the maga would still get an apparent age increase on a roll of 7, 6, or 5 (-5, -6, and -7) and would have a ~48%+** chance of hitting an apparent age increase on a 1 (subsequent rolls of 3, 8, 9, 10 or potentially another 1 would do it [and worse]).
So even with a very strong ritual, there's always a good chance of hitting an apparent age increase. Magi aren't guaranteed to hit those numbers, but over the course of many years, they are much more likely to hit them. And if that probability isn't high enough, you can extend the range from -4 to -7 or -3 to -7, which would result in the magi apparently aging at about half speed.
* Since Aging rolls are stressed with no botch, the potential for an apparent aging increase will change based on what result a 1 on the die would normally yield. However, this is also true with a lot of results on the standard table due to how exploding dice work.
** I'm not doing the exact math on this, but a quadrupled roll of 4 or higher will always result in an apparent aging increase (at least) and an another 1, in addition to being extraordinarily unlucky, would guarantee Crisis.