Even though my table saw and Miter saw still work great, they are both old. When do you know when to replace them?
When they die or when you want to upgrade.
Well for these tools, as long as you can getaway with just replacing old and dull blades, I think you're all good no matter how old they are. But when the machine acts up all the time or it kinda puts safety at risk (e.g. blades flying out), you gotta replace them.