The tire surface normally crack after 3 - 5 years. In my experience, the crack is only at the surface. As long as you have a spare tire in your trunk, and has sufficient air, you can still use the cracked tire.
Since you don't post your car model. I don't want to provide false info. This comment is based on following assumption:
1. It is not heavy vehicle like full size SUV, or 2 tons car, bus, truck
2. Not putting full load in the car
3. Always maintain proper tire pressure with monthly inspection (ignorant driver takes note)
In short,
1. careless drivers please replace
2. ignorant drivers please replace
3. drivers who check their car condition monthly can keep it (esp tire pressure)
4. drivers who like to play safe please replace
5. drivers who wants to save money can keep it, but must do step 3
6. drivers who know nothing about car replace replace. I assume you are driver type 1, 2