I know O2 sensors are supposed to slow down with age . I was driving the other day and realized the front sensor in my Honda Accord is over 330,000 miles . I also just got 32 and change MPG on a recent road trip at 80 mph so it seems to be doing what it should.

Our mechanics (Honda Dealership) just run them until the go Tango Uniform...

If the light comes on, then bring it in.  If it ain't broke, don't break it!

BIGGER_HAMMER

Personally I like to replace them at 100k miles.  But, in your case, I would just run it.  I'm surprised that they have lasted this long but at this point, I'd be curious to see how long they last.

That car has been all the way to the moon, and a good chunk of the way back.

Depends, if it's dieing slowly i'd replace it. But for example in a project car of mine (1990 Eagle Talon TSi) some leaded race gas killed my O2 sensor (didn't cycle correctly) with one tank of it. Ran hella good at wide open throttle though! If you know the testing procedure for your vehicle you can test the O2 sensor to see if it is producing the correct operating range of voltage. This all depends on if it's a narrow band or wide band O2 sensor as well. If I recall correctly most narrow band sensors swing between .12 and 1 volt, dont quote me though as it's been a while. Most wide bands operate between 0-5 volts. The higher the voltage the leaner the fuel mixture (on the car's i'm used to working on anyway)

With narrow band the computer is constantly adjusting even at a continuous load, throttle, speed etc. Because of the way the sensor and computer check the A/F mixture, it's literally like a pengillum swinging. At one point it will see the fuel mixture is a hair to rich say .72 volts so it will lean out the mixture, next cycle it checks (were talking fractions of a second) it may be .89 volts and be to lean with the target some where in the middle. This process will continue on for as long as the engine is running. The computer just has specific target air to fuel ratio's for given loads, throttle and airflow (maps). It selects the correct one for the given moment and swings around it. Again that's narrow band and OBD I and OBD II computers. Some newer vehicles that come with widebands, i'm not sure. But a wide band reports the exact a/f ratio voltage continuously and over a much wider range, giving it much higher resolution. Great for tuning both open and closed loop fuel maps.

I like to think I run a pretty good maintenance program .

The rear O2 has been replaced once

I only replace non-critical parts when they fail. Your Honda can live without an O2 sensor.

Yes but it will kill the cat and the mileage will drop.

in your case I would go for broke, for most people you may want to consider how long you are going to keep the car.

For example, if your car has 90k miles and you plan to keep it until 150k miles.  However, from experience personal or online, you know they will die by 120k miles.   Well, if you suspect they are not optimal, why let the next owner get all the benefits of new O2 sensors.   Might as well get new ones and enjoy them early.  

I use the same logic for a lot of replaceable items, esp. plugs and shocks.

You get an engine light as soon as there's an out-of-bounds reading for more than a key-cycle. That's not going to kill the catalytic converter. At least, it never has on me.

ETA:
O2 sensors die gradually. The readings become less accurate as the carbon cakes into them. If you're considering replacing one, first dunk/soak it into carb cleaner, it'll buy you a couple more months on it. (some people say this kills them, but just don't do it hot.)

I run them till I catch a code.