Signs of wear and tear that mean time to replace parts....

[GoldenLamprey]

^ wot he said.

Some tyres do have wear indicators, Continental GP4000s, definitely. They are just dimples in a couple of places, and if they are flat, time to replace.

 

[SkipdiverJohn]

Nuisance punctures will become unbearable before a tyre wears to the point of structural failure. The front tyre of my hack Apollo MTB I've just discovered is flat. The tyre is ratty as f***, most of the tread has gone off the centre part and there's multiple small surface cracks. My intention since March lockdown has been to keep running it until the next time it flatted then skip it at the same time as fixing the puncture. Now it has flatted, I'll bin it and choose a replacement from my pile of slightly less ratty salvaged knobbly MTB tyres and the process starts all over again.... My stock of spares isn't even going down as I keep finding more faster than I can wear them out.

 

[Blue Hills]

Nuisance punctures will become unbearable before a tyre wears to the point of structural failure. The front tyre of my hack Apollo MTB I've just discovered is flat. The tyre is ratty as f***, most of the tread has gone off the centre part and there's multiple small surface cracks. My intention since March lockdown has been to keep running it until the next time it flatted then skip it at the same time as fixing the puncture. Now it has flatted, I'll bin it and choose a replacement from my pile of slightly less ratty salvaged knobbly MTB tyres and the process starts all over again.... My stock of spares isn't even going down as I keep finding more faster than I can wear them out.

Pretty sure some of the more than partly worn tyres are 15 years old. I too will use them until repeated punctures.
I always put newish decent tyres on for a tour though.

 

[SkipdiverJohn]

Pretty sure some of the more than partly worn tyres are 15 years old. I too will use them until repeated punctures.
I always put newish decent tyres on for a tour though.

I don't care how old a tyre is, so long as it is structurally sound, and with the use of nylon rather than cotton threads in the plies, age and damp storage doesn't matter. I have a couple of spare Michelin World Tour tyres that came off an 88/89 bike and a couple more chinese specials from the mid-90's. So long as the rubber doesn't start to disintegrate I'll happily use old tyres until they wear out.
Having said that, you will appreciate most of my mileage is relatively local and short distance, so a puncture or tyre failure is just an annoying inconvenience. On the bikes I use for longer rides, I am rather fussy about my tyres, mostly running Schwalbe Marathon/Delta Cruiser+ - which get checked regularly for embedded objects & damage.