The only rats guaranteed to be free of the mycoplasma bacteria are labratory rats kept in a perfectly sterile environment. For this reason, mycoplasma pulmonis is the most common cause of respiratory infections in rats. Although there are several other bacteria that can cause the same symptoms, the usual culprit is mycoplasma. When a rat gets old, or gets stressed, the mycoplasma bacteria has a chance to take hold and multiply.
Pneumonia is when the infection gets into the lungs.
If your rat is sneezing more than usual, listen closely to his breathing. If the nasal passages and chest sound clear, keep a very close watch on him, but it shouldn't be necessary to start treatment unless he starts acting more tired than usual, or stops eating or drinking. Allergies and dust are also possible causes of sneezing. If there is a clicking sound from the sinuses, he is congested. This could progress further, so treatment would be a good precaution at this point.