OK, first of all: I can't believe you quote Calvin & Hobbes in your sig. I love that comic strip! Go Spaceman Spiff!!
I like to give a set amount of DPs per level and this last time it was 100 a level too. They don't need more DPs at higher levels, I feel that a method for being able to increase skills a little faster would be better.
What I mean is, say you are a 30th level magician and you want to pick some cool Self Control skill, like Stunned Removal (it really sucks to have your spell disrupted, I mean really, so what if you were stabbed in the kidney - its my spell and I want to cast it!! Mooooooommmmmmmm!!!!). Now you have a Self Control progression of 6. That means that only one rank per level could you gain. Let's change that by enforcing stricter training times (we all fudge on those, come on, you know you do......).
Say you cap it at a certain percentage of each level's DPs, we'll go with 30 pts to keep the math nice and easy. That means that when he gets to 31st level, he can pick up 5 ranks of Stunned Removal, but the training time could be months, maybe even up to a year. That does two things. Allows the character to gain a skill that they can actually see some use from immediately and gets rid of the too powerful 25 year old.
The 30th level magician should be getting on their in years, not still popping zits. (Unless, of course, he fumbled a spell and got whole body acne as a result! Sucks to be you.)
The maximum ranks I would allow them to get to with this method, would be equal to their level. This reflects that it is no longer a new skill, but one they are more familiar with so progression would slow down. For many skills the initial learning is the fastest, but then getting better after that initial push is much slower.