When to replace appliances: Proactive or reactive?

[Petra M.]

Whats your position? Replace things before they break (proactive) or don't fix what's not broken (reactive)? Also, when do you know if something is nearing end of life? 

What's your average life span for the following:

- AC and air handler/furnace (replace both at the same time?)

- Water heater

- Roof

- Fridge

- other things?

I have some pretty old hvac and water heater in one of my properties that are original from when it was built in 1999. Things work but wondering whether I should be proactive or just wait until it fails.