Nice catch, Damon! Actually, as long as Al is always an observer, it really doesn't matter how old he is, because his position as a hologram doesn't require any of the daunting physical tasks that Sam has to endure while leaping. For Sam on the other hand, if he was Al's age, which according to Damon would be 72 in "A Bold Leap Forward" (assuming it comes out in 2006), it just wouldn't be plausible.
At 53 obviously it's easier to kick butt than 72 :b . Even in the "Quantum Leap: The Complete A-Z" book by Louis Chunovic, there's a page that has Al's stats and it says the year of his birth is "Unknown, estimate 1940" if I remember correctly. Then again the copy I have is not the updated version after the series had ended so Al's age probably wasn't established yet at that point.