Are seven year old tyres still safe?

I have a 7 year old Focus with the original back tyres. Tread depth is just over 4 mm. is it worth renewing these now due to their age or wait until they wear down to 3mm?

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...