This is prolly a better question for Kenzi, but I’ll take a stab at it!
We used to think that bacteria didn’t age because they basically just keep on dividing and creating more generations. But turns out they get old, as well and start to slow down their dividing as they reach the ends of their lives. Precisely how old they are when this happens depends on the kind of bacteria, but some microbes can live for hundreds of years! We’ve even found evidence of 3.5 BILLION year old bacterial ecosystems in Australia, so we know that we’ve had bacteria on the earth for at least that long (though I don’t think they were still alive, though).