A technician told me my Day/Night forced air heater needs to be replaced because it is 15 years old and has cracks in the heat exchanger. Another company came out and examined it and couldn't find any cracks. Should I replace this heater because of age?

Have the first company prove the cracks are there.
It could be a scare tactic to get you to buy a new furnace

OR

Your furnace could have a serious problem.

There is no cut and dry answer to your question because you are getting conflicting information.

It is easy to rule on the safe side and replace the furnace but if you really do not have the money it can be a financial burden

Furnaces can last a lot longer than 15 years but you never know. I have seen them go bad in 2 years and last 40 or more.