Do I need to replace both rear tyres on my car at the same time?

I bought a second-hand Range Rover Velar where three of the tyres have 7mm of tread left but the offside rear tyre is down to 3mm. The garage says this one ideally needs replacing but there is no problem...

How old can tyres be before they need replacing?

When should I change tyres on my car? Is there a maximum age they can be?

Do tyres have to match?

My front driver's side tyre on my Toyota RAV4 has a slow puncture and I'm wondering how important is it to brand match, tread match etc. tyres when you replace a tyre? Should I move the two rear original...