The accepted life is 100k miles. It's a fair bet a sensor is worn out if the vehicle is this age.

The CEL will not always come on - it has to go far out of range or be open for this to happen.

When the front (regulating) o2 sensor goes bad, your gas mileage goes down a bit.

The rear one isn't all that important, it's only compared with the front sensor reading to make sure the cat is working.