15 years old is nothing. I have electronic gear built by Tektronix and HP (the real instrument company before they became a PC printer company) that was built in the late 60's and 70's that still meets original specs today.
I also have a recently re-furbed Spectral DMC-10 phono preamp from the mid-80s that sounds just wonderful. I did re-cap this and rebuilt the PS.
With a power amp, the number of hours on it will have an impact due to the heat, as heat is a killer of electronic devices. If it sat off for many years, this is better than someone running the amp 24/7. Unlike what some will tell you, running the gear 24/7 is not the best for the components. There is also a possibility that voltage transients from power line surges can weaken components like bridge diodes and perhaps power transistors. Sometimes you will hear "pop-corn" noise or hum from the input stages (the input transistors). If there are no hum problems or noise, the amp may be in fine shape.
Regarding re-capping, be sure you can get replacement caps in the configuration you need them in. Some older gear used electrolytic caps that were "twist lock" style that are virtually un-obtanium these days. The computer grade electrolytics that have screw terminals may also be difficult to find. This may mean custom kluging to get a modern Panasonic cap to fit a board with screw terminals. A good example is Parasound HA 2200ii which used screw terminal PS caps, which are difficult to find except though Parasound (read EXPENSIVE). So you need to take a good look inside the amps before you commit, and then do some research on the replacements.
And, if you think you can find the older caps on EBay, you may but they will be just as old as the ones you have in the first place. If you can, use upgraded commercial grade (105 C rating) electrolytics for the replacements.