Possibly younger than that.
Age and growth are not well-documented for alligator gar, partly because
traditional methods of age determination do not work. Gar scales do not
exhibit conventional growth rings and other non-destructuve techniques
(scale sections, rays) have not yet been shown to reliably predict age.
Otoliths, I believe, work but require sacrifice of the fish and large
numbers of specimens are needed to generate reliable age models.
There are two other problems with large, long-lived fishes. Growth rings on
old specimens are tough to count because growth slows down as fish age
causing recent rings to become crowded. Also, some individuals grow much
faster than others so there is a high degree of variability in size for fish
of any given age class.
Take a look at this site which has Elise Irwin's age-growth data for Alabama
alligator gar:
http://www.ag.auburn.edu/aaes/communications/highlights/winter00/fishtales.h
tml
It suggests that a fish of that length (approx 8 ft) could be approx 30 or
50 years old. The Mississippi River fish was substantially heavier, though,
than the Alabama specimens of comparable length so its anybody's guess until
someone examines an otolith.
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