Is the decline of fluid IQ with age less severe for professors and scientists than it is for other people?
Seattle Longitudinal Study:

You might want to have a read about The Seattle Longitudinal Study of Adult Intelligence.
There's a summary of the study on this website.
To quote the website:


  The Seattle Longitudinal Study of Adult Intelligence has followed a
  group of more than 5000 people for well over four decades. The program
  began in 1956 and participants have been tested across a whole gamut
  of mental and physical abilities at seven year intervals since that
  date.


With regards to preventing cognitive decline the website suggests that


  The following variables may reduce the risk of cognitive decline in old age:
  
  
  absence of chronic diseases
  a complex and intellectually stimulating environment
  a flexible personality style at mid-life
  high intellectual status of spouse
  maintenance of high levels of perceptual processing speed
  


If these general points are true, then presumably an argument could be made that scientists and professors have more complex and intellectually stimulating environments. However, the age related declines may be most pronounced in what are potentially post retirement ages (e.g., 60 or 70 +), where the environment would be different.

Some general points


Professors and scientists have some of the highest levels of intelligence among the professions (see this table based on this working paper by Hauser, 2002, PDF). Thus, if everyone did decline cognitively at an identical rate, people from such professions would still tend to have higher intelligence (e.g., a 10-point decline from  130 is 120, versus 90 to 80). So, people with initially high intelligence even with decline can still be intelligent. However, this initially high intelligence is most likely due to genetics or some undefined source than to the stimulation of the profession itself.
Thus, indirectly, the above points (high intelligence of scientists and professors) and potentially environmentally stimulating environments suggest that they will be more intelligent at older age. But my guess is that most of that effect will be due to base line differences in intelligence.