Way back in the dawn of time just as NorCon PernMush (the original) was starting up, I helped code for a simple ElfQuest game called TwoMoonsMush. We only had elven magic as skills and the time scale was long enough that we created a learning system where regardless of anything else, given enough time your, say, Fire Magic would increase. The curve was diminishing returns, so the older you were the slower that would go. I can see this working with a system that got you more starting points for later age to simulate the learning you missed before creating the player.
Anyhow, it worked remarkably well, and could be enhanced by occasional RP points. This being based on a Chaosium system, we would give people a random very low amount if they showed that they learned something about their magic by failing, or something like that. It was also a percentile system, so people would see the small point increase over time, whether it was RL months or years.
This is the only time I have ever applied calculus to my real life.