Personally I like to inspect my tires off the car once a year. Good time to clean the entire wheel, check the braking system while I'm at it. Irrespective of age or miles, I look for cracking, uneven wear etc. Found an embedded nail or screw a few times over the years. Usually I wear out the tread before the sidewalls deteriorate and I hate to drive misalignment worn tires, even if slight. Bottom line; replace based on condition.