One thing with intelligence would be vocabularly as you cannot think or imagine without the words to do so. Other things being equal (normal brain versus idiot savant) one's "intelligence" would go up as their vocabulary went up.

If I'm a high school dropout mechanic and I want to be my own boss, I'd have to be able to imagine/master bookkeeping/accounting and advertising and other business subjects and parts department, hiring employees and just the whole "overview" of how a business operated. If I had never been exposed to that in any way/shape/form I wouldn't know "how" to think about it. There would be a foggy void at some point. But if I learned those words and what they meant, did some reading or apprenticed in a locally owned shop where the owner helped me learn other aspects of the business, that would increase my intelligence/knowledge.

I think that's where people getting wiser as they get older comes in; the experience of living teaches us new things whether we want to learn them or not.

Some of how much one could learn, the "limit" would have to do with time and background. I think life is like a pyramid of sorts so one wants as big a base as one can get so one can get higher. The more experience one has the more likely one is going to have ways and means to have alternate ways to think about things and come up with more ideas on how to solve a problem. One has known really bright professors who live in ivory towers and don't have much "life" experience because they were studying books but one also knows "practical" people who are similarly "intelligent" from having experienced many different scenarios and had adequate schooling/words to make sense of their surroundings and those scenarios. I'm not talking about "street smarts" they are usually not practical other than in keeping a person "alive" in a hostile environment and tend to enforce the belief that all environments are hostile. But there are engineers with similar degrees but one is a "design" engineer, inventing real hardware/software solutions and there is an "administrative" engineer, being a paper pusher. I want the design engineer with me on the desert island, Gilligan's "Professor", not the PhD, esoteric, how many angels dance on the head of a pin guy.

Mental clarity seems simple to think about :-) at first because of the word "clarity". Intelligence and fogginess don't go together for me. Mental clarity problems would indicate a breakdown in the physical, chemical, whatever for me; a lacking of resources, whereas the edge of intelligence would have to do with the edge of experience. The rate/ability of learning is an inherited thing, I think. I watched as my stepsister's children struggled in school whereas my brothers and I went through school without breathing hard. That's only perhaps one type of intelligence though and it's possible that some of the other types compensate and mold the experience in another way. But one's ability to grasp experience quickly would make a difference I would think.