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?

Are there guidelines on tyre tread depth for an MoT?

My partner has just bought an Approved Used Toyota Yaris Hybrid from a Toyota main dealer on my recommendation. The MoT certificate was issued on the day of collection with no advisories. I have noticed...