Is the decline of fluid IQ with age less severe for professors and scientists than it is for other people? This article in Time magazine suggests that engaging in challenging mental tasks may be associated with lower rates of mental decline in older adults.
The article states that:


  In one study, presented last year at the Cognitive Neuroscientist
  Society's annual meeting, psychologist and neuroscientist Helena
  Westerberg of the Karolinska Institute in Stockholm compared the
  cognitive abilities of 45 young adults (average age 25) with those of
  55 older adults (average age 65). She found that after five weeks of
  computerized training on tasks ranging from reproducing a series of
  light flashes to repeating digits in the opposite order that they were
  given, the older group was able to reach the same level of working
  memory, attention and reaction time that the younger group had at the
  outset. (Notably, the younger group had even greater improvements by
  the end of the training period.) "The program is always pushing them
  to do better," says Westerberg, who notes that an advantage of
  video-game training is that the programs' difficulty level continually
  adjusts upward to match players' evolving abilities. "They have
  feedback and can see their scores."


This might suggest that scientists and professors might have less decline in fluid IQ with age. Is there any support for this idea?
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.