In humans, intelligence manifests as memory, spatial and verbal reasoning, pattern recognition, etc. What is so interesting about IQ and g (the general factor) is that all of these abilities trend together. A score in one area is a good prediction of the score in another area. There is no reason why that must be the case a priori, and LLMs are a great example of an intelligent system which is much better at recalling information than it is at reasoning.
Human aging doesn't seem to affect all of these abilities uniformly. e.g. Everyone seems to complain about memory the most (and that matches my experience), but I've been pleasantly surprised how well neuroplasticity and pattern recognition have held.