The typical lifespan for tires on most vehicles is 5 years or every 50,000 miles depending on how often you drive the vehicle and how many miles you put on the vehicle.
The tread if worn down enough will not be very deep and when the tire tread wears down the tires should be replaced.
Also if the tires are cracking or the belts are separating or there are any cracks in the tread area or sidewall the tire should be replaced even if the tires have good tread.
After 5 years the tires are aging and the rubber of the tires are getting old and you should look into replacing them especially if you're going on a long road trip with the vehicle.
I've had some tires last 10 years and even 17 years but those tires are on some vehicles that I've had that don't see a lot of highway driving.
A tire can blowout and cause a rollover and cause death or serious injury so replace the tires when they are older than 5 or 10 years to be safe.