Gas and Oil Home Heating Furnaces - gas furnace replacement

When should I consider replaceing my gas forced air furnace? Unsure as to the age, wondering if the serial numbers would give a clue. I suspect it's 15 to 20 yrs old. It's a Rheem RGAA-140C.
What nessesitates replacement?
Cracked heat exchanger?
Rusted burners?
Can and how would I inspect the heat exchanger for example and would carbon monoxide in the home be indicative of a cracked heat exchanger or could it be something else?

as you furnace ages, the heat exchanger becomes thinner and thinner due to rust flaking off and often covering the burner. as the exchanger expands and contracts, it is more and more likely to crack the thinner it gets. CO in the home is a sifn of cracked exchanger, but could also be caused by leaks in the flue. CO is a product of incomplete combustion, the cleaner your burn, the less CO produced. to inspect your exchanger, remove the burners if possible, shine a powerful light up into the cavity, use a mirror if needed. if you furnace is 15 to 20 yrs old, i would replace it with a more efficient unit. this would put your mind to rest as to safety, and the efficiency payback would be noticeable. if the evap coil is original, replace it as well at this time, it would be cheaper.

Thanks for the info. I'll see if I can get the burners out and look at the heat exchanger for cracking. Thanks again.