I think it comes down to how a boat is taken care of and used. Two guys can have very similar boats for sale but one may be worth far more than the other. It comes down to what your getting for your dollar.
If one boat spent it's life under a tree with a piece of plywood covering the engine, rarely sees an oil change, and is abused in the woods too, it's not worth as much to me as the one that was taken care of and maintained properly.
I have an excellent 4 cylinder boat for sale for the same money you might be able to buy a six cylinder for, but cahces are your gonna spend time under the hood in the marsh. When I stop to drink some coffe at 1:00 am in the morning and I'm 12 miles from the launch, when I switch on the mags and push that little black button I want that thing to light.
Getting back to the subject, I don't think depreciation in value of an air boat necessarily has anything to do with age. I don't think Cowboy would take $1,000,000.00 for his 1972 "Woods Pussy".