Do doctors use any special criteria when deciding whether or not an elderly patient should undergo chemotherapy?

Case-by-case basis: Every situation is different, and every decision for something as serious as chemotherapy is made on a case-by-case basis. The basic question is this: is there a realistic chance that the chemo will prolong the patient's life, and improve the quality of life, without making things worse? Or to put it another way: do the potential benefits of the proposed chemo treatment outweigh its risks?

Yes: There is always an evaluation to decide whether chemotherapy will have benefit at any age. The same goes for the elderly. In general, something called "performance status" is a major deciding factor. Also one must evaluate the potential benefits of chemotherapy based upon a person's age and expected longevity. Surprisingly, many elderly patients with a good performance status do quite well.