How long do tires last or need to be replaced by?

I have a used car that is 6 years old, and when I bought nit the tires were never replaced. They look ok to me, but being I don't know very much about tires unless the threads look beaten, I dont know when to replace them. It was from a car rental place, then had one owner. I appreciate any advice.

How long do tires last or need to be replaced by?
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