Fluid Intelligence is strongly associated with natural abilities such as IQ, which is Genetic. It is our ability to perform complex tasks and solve problems, which is the function of IQ which exists at birth.
Fluid Intelligence appears to drift away after our early adult years. While Crystallized Intelligence is our learned Intelligence through experience and education, this lasts longer in the elderly than Fluid Intelligence.
Which is why your grandfather may want you to help him sort out how to program his new television, as his fluid intelligence (problem solving skills) are no longer up to the task.
I believe that many factors effect a person's intellect and that the majority of them are not genetic or developmental. I believe the current understanding of intelligence needs revision as well as the constant degrade of intelligence suggested and the leveling off at a certain age do not properly correlate with anything other than flawed testing methods versus actual performance based criteria.