I have tried to read through a lot of the posts, to see if this has been answered already, but I apologize if I missed a response.
We have lived in our house for over a year now. It was built in 1958, and is a one story ranch house style home. Just in the past two weeks, we have had a reduction in the hot water pressure in the master bath. It now has pressure of about 30-40% what it used to. We have no other issue with low water pressure in any other area of the home, just the master bath shower. The shower head is less than a year old. I don't think it is an issue that the head needs to be cleaned or the cartridge, since the cold water pressure is still strong as ever. The faucet has had no change in water pressure (both equally strong and fine). Is the lack of hot water pressure due to bad hot water line directly to the shower? Or would it be the hot water heater, which by the way IS pretty old.
I appreciate your input. :)

We have no other issue with low water pressure in any other area of the home, just the master bath shower

That should pretty much eliminate problems with water heater.

What kind of tub and shower valve do you have? Two handle or single lever?
Cam you post picture of tub and shower valve?

I am going to assume that you have a one handle shower valve. I don't believe it is the hot water heater, the shower head, or the hot water line from the heater.
It sounds like you may has some trash blocking the hot water inlet port on the cartridge.
Try this. Shut the water off to the valve and open it up. Remove the cartridge and checking clean the inlet ports. Before you reassemble, turn the water back on to flush out the supply. Reassemble and test. Good luck, Tom