This is exactly what I mean. I deliberately left "growth" ambiguous. (Luke is a less interesting character in part because he doesn't really change; this does make him a classic protagonist however.)
Yes, they do tend to show up more. I actually think capability change is more realistic. However, certain games go way overboard. For example, D&D has enormous changes very quickly, which isn't very realistic. GURPS, for example, seems to be much more realistic about the rate of change compared to how a character starts. ArM5 does have improvements costing more and more which helps create slower rates. It also requires seasons for improvement, which is reasonable. Still, it's on the fast side.
The thing none of the games I've seen tend to do well is forgetting. Realistically, without continual practice skills decline. I keep learning a lot as I go, even well out of my 20s. However, even though I've been teaching physics for a long time, I haven't been doing the really high-end sort of stuff I did in graduate school. So there are lots of areas I understand the pieces of but have lost the specifics and forgotten the mathematical tricks for. Perhaps more obviously: juggling. When I was about 21 I could handle six balls in several patterns. I have juggled less in the last two decades than I would do in a week or two way back when. Now I struggle with five balls. At the same time, I could barely dance at the age of 21 and eventually became a national champion. While improving in some areas other areas were neglected.
It's bound to happen to you poor Hermetic Dancers. Dancing always did have more standard botch dice than casting spells. A few bad Twilights later, and you've lost a bunch of knowledge and gotten Flaws to make you slowly forget more. Truly heartbreaking, for a life of study and prestige to be watered down by the cruel grip of Twilight!
Not saying your point isn't true, of course. ^_^' I don't think a skill forgetting system would be very popular. Maybe as an optional rule in a sidebar? But then, forget magic item crafting. Or really any possible activity a player might partake in other than reading every book they have all the time to avoid losing their xp in skills. Because players.
A nice addition to other, more simulationist, games, but not so much Ars Magica. Losing skills gradually (rather than all at once, through, say, Twilight) does almost nothing to help the narrative progress in good and/or interesting ways, regardless of how realistic it is.
Ya, not saying it would be popular. Just that it is quite realistic for people to learn, though more slowly when older. It's not the learning part that isn't so realistic as the not forgetting part.
Somewhat related to the forgetting abilities, in the Price of Freedom saga I established a House Rule about allowing Area Lore to mutate from one area to another. It made no sense to me that you would keep an up-to-date knowledge of an area you no longer lived in, but I didn't want to simply have that knowledge disappear.
This way, I figured it would actually encourage players to invest some of their character's xp into Area Lore, knowing that they could change it along the way based on where their character now lives.
It goes like this:
Also, knowledge and skills can become obsolete. A noble in the court of King Gilgamesh might have problems with etiquette in Venice...
Also not realistic is the way some people simply learn anything faster than others.
Sure it makes sense. Some people learn better in different ways. Some people are exceptional at remembering what they read, some people learn best by hearing other people explain things, and some people have a knack for getting things down by tinkering with them themselves. That happens IRL; some people are just exceptional learners. The xp-boosting (not granting, boosting) Virtues represent memorization, really; where another character needs four seasons of reading to memorize all the information a book can provide, your character only needs three, because he doesn't need to pore over each section as long to memorize all the relevant information.
I've put some amount of thought into this, sadly.
It's realistic, but every way I've found to model it, was more effort than it was worth, unless I can be garanteed the process is automated.
Meaning play would be locked into a piece of code. no more p'n'p.
Your milage may vary ofcourse.
One can handwave and just say that loss of Int from aging rolls represents memory loss.