Although it is not uncommon to get a flat after sometime, regular flats mean that your tire need replacement. Tires should not go flat completely or be unable to hold air. Before you replace you tires, check the rims to ensure that the seal around the tires do not let out air.
If the treads go low, or there are no treads, your tires lose their traction. Such tires are prone to bursting when you are driving. Bad tires can also give you bad mileage. This is especially so when they cannot retain air for a long time. You could lose up to 5 mpg if your tires lose so much air after inflation. Tire bumps are also signs that you need new tires. This usually happens after a minor accident, and they are risky to drive around with.
Cracks and other weathering signs are also indications that you need new tires.