I would be more inclined to believe any differences in intelligence, if there really are any, are due to the environment in which the child was raised, rather than genetic advantages and disadvantages. In today's world, I would guess that children born to 50 year old men are being born into a family where the wife is not the father's first wife, and the father is not as likely to be as dedicated a father as the younger fathers plan to be. This is a sweeping generalization, but I would wager that for those 50 year-olds who are remarried (once, twice, or more), by the fact that they did not care to keep the family intact the first time (excluding cases where the wife died), these men and their current wives would be less likely to provide a nurturing environment compared to a young father.
A sweeping generalization, but my gut feeling, based on the likely fact that many of those 50 year old fathers are remarried once, twice, or more, indicating to me that they are not as good a father as the SOME of the 20 year olds will be.
Also, statistics on society's accepted geniuses do not seem to show a correlation between fathers' ages and the intelligence of the offspring.
Here is a list I have compiled. IQ scores may be disputed, but the intelligence of these people generally are not.
Genius IQ Father's age
------ -- ------------
Einstein about 160 31
Galileo 185 44
==========================================
Bobby Fischer 187 Likely father, Paul nemenyi: 48
Birth certificate father, Hans-Gerhardt Fischer: 35
http://www.nndb.com/people/455/000024383/
Madame de Stael 180 33
Ludwig Wittgenstein 190 42
Blaise Pascal 195 35
Gottfried Wilhelm Leibniz 205 49
John Stuart Mill 200 33
Emanuel Swedenborg 205 34
Johann Wolfgang von Goethe 210 39
As you can see, one of the highest IQ's, Leibniz's, had a father who as 49 years old.
Einstein's, considered the lowest at 160, had a father of 31, the youngest in this list.
So this is opposite the study's findings. However, I don't really see a correlation overall.
With the study's difference of only 3 IQ points on average, I am inclined to believe that age makes no difference in the intelligence of their offspring, especially in those families that provide a nurturing environment, regardless of the parents' ages.