Tires- How old is too old??

[rbr]

Hi all=
We are looking at a one owner 1990 model 29' unit that had new Carlyle tires put on in 1998, was then parked outdoors and has not moved since.
Reportedly they look "a little discolored" ie. faded, but no visible checking/cracking. I plan to repack the bearings and probably replace the brakes before pulling it home if I buy, but should I replace the tires too?
It will be about a 5-hour haul down the interstate in a pretty rural area.
Thanks, Bruce