Is it harder to get medical treatment for people over 75 years old?

Most insurances will not cover the outlandish cost of a PET scan because it won't make any difference. Both Parkinson's and dementia are terminal conditions. Dementia, however, must be diagnosed. It is not in itself a disease nor a true diagnosis. It represents a collection of symptoms caused by some underlying condition or disease. It has over 70 causes so you must find out the cause so you know what you're dealing with. Today with all the tools a physician has at his/her disposal, a PET scan isn't necessary. MRIs, blood work, cognitive tests, family history and psychological tests can be administered to determine the cause of the dementia with almost 95% accuracy. So forget the PET scan and have her PCP refer her to a gerontologist or neuropsychologist.  A PCP is not trained to make this diagnosis.

My mother survived both breast cancer and malignant melanoma in two locations. She was never given scans of any kind. She was over 70 with her cancer diagnoses.

I had breast cancer in my late 40s. I have NEVER had scans other than a lung scan when a chest xray showed a suspicious shadow. I stopped all mammograms at age 70 (retired RN) by my choice.
And the answer is that yes, there are recommendations for tests and age. And in fact colonoscopies past age 75 not recommended under current guidelines either. You can look up guidelines online; just type in, for instance, "CDC guidelines colonoscopy".
Testing is very expensive, it often leads to a lot of anxiety and it often leads to false positives and further testing.
Both Dr. Dean Edell in his book Eat, Drink, and be Merry, and Barbara Ehrenreich in her book Natural Causes talk about the over testing and over treatment of the elderly in our country, and the torment it causes for them before they are allowed finally to be at peace. And of course the books of Dr. Atul Gawande including his Being Mortal, are not only famous but infamous.
As the only advocate now for people who cannot make these decisions for themselves I would consider carefully what you knew their decisions, when they were competent to make them, might have been. Hopefully you did discuss things with them at that time.
If you truly wish certain tests to be done, and there are SYMPTOMS that the doctor agrees could indicate a need for them, and you wish to be more aggressive in testing and treating rather than less, then revisit this with the doctor. It is my opinion that putting someone elderly through aggressive treatment for say a diagnosis of metastasis to the bones, is basically a torment, but we all have our own opinions; once educated about all contingencies, we have to make our own conclusions. For myself, in your cases and in your place, I would follow the PCP guidelines, but I am not you.

If your mother has cancer, are you going to treat it? She has Parkinson’s and dementia. Can she even tolerate a PET scan? Is your goal to prolong her life no matter how bad her quality of life is? If she has cancer, are you going to subject her to chemo or radiation? What is your end goal here?

Medicare will only pay for a PET scan if it’s ordered by a dr and deemed medically necessary.

My mother is just under 94 and hasn't been refused various tests over the years. They have to be warranted, however, before a doctor will agree to write an order for them. A PET scan uses contrast dye which can be dangerous for some people, especially the older we get. There are other, less invasive tests which your mother can be given to find out why she's in so much pain she can't get off the sofa. It's not necessarily bone cancer at play here! I have bad lumbar arthritis and degenerative disc disease which renders me in pain and immobile lots of times. That diagnosis was arrived at with simple xrays which your mom's PCP would likely agree to, at least as a first step, you know? If the xray shows something significant, maybe THEN she'll write an order for a PET scan or the next test down the line.

If you have questions or concerns about your mother's care or why certain tests are being disallowed, why not ask the doctor up front? She likely would have told you why the PET scan was 'frowned upon' and you would have had an answer! Be aggressive with these doctors! Get the answers you need, you're entitled to, and so is your mom!

Good luck!