Depends on type of tube. Tubes degrade gradually with use, but will "work" considerably longer than their ideal useage time. So you could have a tube working 5-10 years that musically degraded after 2-3 years. The only way to know is to try new tubes and see if you hear a difference. If not, you have a back up set of tubes.
Replacing tubes on used equipment bought on A'gon
Over the last 18 months, I have bought through Audiogon a CD player, preamp, and amp...all tubed and used. They are not giving me any trouble that I know of. As I am a newbie to tubes, is there any merit or need in replacing the tubes on used equipment that is purchased since I don't know how they sounded as new? Thanks.