Skill use also plays into that to some degree. Your level will determine (more or less) your capacity to learn skills, and usage of the skills will determine how you put that capacity into use.
Skills which have not been recently used will gradually lose experience, but only so much. A skill will never fall below 90% of the total amount of experience you have ever had in that skill from degeneration (although counterbalancing can reduce it lower than that). This, combined with the max xp that a skill can have in it, means that a character, even a fairly young character, who practices making horse shoes all the time can keep his horse shoe making skill at maximum. A character, even a very old character, who does not practice making horse shoes all the time will not quite be at maximum horseshoe making skill. The younger character can build a reputation as the best horseshoe maker and keep it, because it's not worth the older character's time to spend making horseshoes. When even the most powerful characters want a basic horseshoe, your character can be the best in the area. Of course, you have to pick your niche so there aren't a hundred other people in your area that are just as good at you at doing whatever it is you're best at, but that's not too hard since Magicosm has so many options available for you.
Your skill trees will have counter balances. Time and training on skills in some trees reduces skills in other trees. You can juggle proficiency in more skills the more related they are. We think its reasonable for a wizard to be able to slowly master many different forms of magic, just as a knight slowly being able to master many different kinds of weapons. The more total experience you have in a skill tree, the less likely it is to be easily affected by training and activities not related to your skills. It is reasonable for a master wizard to take some "courses" on alchemy, or even fishing. Just like in real life, I can pick up hobbies or take some cooking classes or learn to sew without it really impacting my primary professional capabilities. But if I start to improve, enhance and deepen my knowledge of something like that, I had better do it over a long time, and balance it with continued training in my primary professions, or else my primary profession will suffer. For example, a doctor trains from age 7 to age 26 before he is a doctor (counting all education). He can then start taking cooking lessons, but it might be 30 years before he is a gourmet chef caliber cook AND still a great doctor. He couldn't compress his chef training into 5 years without it seriously impacting his skills as a doctor. But he could easily have several specialties in his primary field.