My CO2 alarm was made in December 2015 when should I replace it ?

It should start beeping when it it supposed to be replaced 

Remove the stack hood from your hot water tank and put the alarm there and see if it gets triggered. If not. replace it.

A professional CO2 alarm would be tested and calibrated every 6months or annually and replaced when it fails to alarm within the defined parameters, such as the infusion of the test concentration of CO2.  Our fire department does theirs once a month. The good-quality meters/alarms last about 3 years.

If you mean CO alarm, about 5 years is the max, but the instructions typically say what is the expected useful lifetime. Newer models simply go into "stupid mode" where they beep like a low battery when the sensor is no longer reliable, and you simple replace the whole thing.

If there's no date visible on it, replace it ASAP.

Even if there is a 10year replacement date, the sensor within can be on a 5 year lifetime.  

carbon monoxide an smoke detectors are ruffly 10 years.  But they also have a lifetime timer/sensor in them eventually it will beep an you won't be able to stop it.  

Roger is correct ...however I would assume you are referring to carbon monoxide (CO) which is dangerous 

I test mine using the car exhaust  (you could use any internal combustion engine)

Start the engine let it run a little put tester in a large plastic bag  (Ziplock type) fill with exhaust ... and leave it for 30-60 seconds ...it should alarm ...If not replace it. 

NEVER, because you don't have a Carbon Dioxide detector.

I don't know the lifespan of your Carbon Monoxide (CO) detector, but I BET that information is stamped somewhere on the unit.

Nuff Sed: You CAN'T be a certified, or even uncertified, Fire Inspector and not know the difference between CO and CO2.

I would not bother to replace it. CO2 is not dangerous. You might feel a bit stuffy with too much CO2 but it is not dangerous. CO2 detectors are rare.