Darth Vader -- Luke, I Have Dementia

The guy who wore the Darth Vader suit in the original "Star Wars" movies says he won't be in the new ones ... because he's suffering from dementia.

David Prowse told the Mirror.uk he'd love to join the original cast with a cameo in the upcoming trilogy ... even though Darth Vader is dead -- but says it would be difficult due to his condition.

The 79-year-old is hoping J.J. Abrams gives him a shot anyway.

Prowse, of course, did all the heavy lifting in Vader's helmet ... while James Earl Jones did the voice.

Now ... SOMEONE PLEASE TELL US who's holding the red lightsaber in the "Force Awakens" trailer?!?