I have an older tiled shower unit in my home. The gentleman who built the home passed away in the 70's, so it is pretty old at this point.
Lately the water turns cold before you are done showering. It does this even if no one else is using water anywhere in the house. I have not detected any leaks and the hot water heater is fine.
It is a one handled control (not a knob) with a round cylindrical disc behind it against the wall. Clockwise to HOT, counterclockwise to COLD. Sometimes if you get it just right you can get the hot water back flowing, but I've noticed to do that you have to turn it back to cold.
I'm thinking that the control is simply worn and needs replacing, but are there other things I should check before having it replaced? Or is 'old and worn' the most likely cause?
Thanks in advance.

You guesstimation that its worn out is likely valid. However, most can be rebuilt which is frequently desirable over tearing into a wall. One handers are most frequently cartrdiges or ball valves. Any chance your valve looks anything like this Moen 2357 Chateau Single Handle Tub & Shower Faucet Polished Chrome - eFaucets.com Château was one of Moens oldest and best like lines even when your place was built.

I have tried unsuccessfullly to attach a picture of the actual control. That one was very similar only with a 4-5 inch long handle instead of a rounded knob. I did look close enough to see that it is a "Standard" I'm guessing American Standard? Fixture. I'll try again to get the picture to load.

Try this site, enter and register to creat an account then search for your valve. Standards are also rebuildable. American Standard - Care & Repair

Will do, Thanks for the help.