Is intelligence variable? Can it be gained or lost?

This question runs the risk of boiling down to semantics, but I’m mostly looking for opinions here.

I’ve always thought of intelligence as being one’s capacity for learning, while knowledge is the sum total of learned information. Thinking this way, intelligence might be analogous to the total storage space on a computer, which is fixed. Knowledge increases as that space gets filled up with “stuff.”

But intelligence is also relative to age; obviously we expect different things from a highly intelligent five year old versus a forty year old. And I think a good indicator of intelligence is one’s approach to solving problems; not the ability to solve a problem per se, since that might be a learned skill and considered knowledge, but the logic and methodology used to approach a problem. Can these skills be improved upon, and if so, is that person becoming more intelligent, or just honing their natural intelligence?

What do you think intelligence is, and is it a variable quality?