At what age should you replace tyres?

I have a 2015 Honda CR-V which still has its original tyres. They passed easily at the last MoT with a tread of between 3-4mm. Externally there is no damage. I recall reading somewhere that old tyres...

Do spare tyres need changing regularly?

I've read that tyres need changing as they age even if the mileage is low and tread is ok. Does the same apply for spare tyres (or space saver ones)? I was just curious as while I think it's checked at...