Some macs like 7200> have a build date on a sticker at the back of the CPU. I can recall that a 5300 (Directors Edition) we owned had a build date and hours used somewhere in the system profiler.

Macs and PCs have much the same bits inside and unless it can be argued that special build PCs have used cheaper bits, both type of machine should have a similar lifetime. If a Mac or a PC was not connected to the net, having to keep up with software is not an issue.

Kev