It could be possible that the HD has become fragmented, thus performance of the Mac will seem to drop as time goes buy. Also after a couple of years, lots of software could have been installed, then removed, but leaving "legacy" extensions and control panel in the System Folder which also become a burden on performance.
So, I would say; Yes a clean install after 5 years would be nice going. 🙂
Maybe a different OS on it? Which Mac is it? How much RAM?