WARNING: spoilers for Star Wars: The Last Jedi ahead!
Although Star Wars: The Last Jedi begins by depicting Luke Skywalker as a crusty old hermit with blue milk in his beard, toward the end of the film we see Luke seemingly appear as his younger self as he faces down Kylo Ren on the surface of Crait. Not original trilogy young mind you, but with a well-kept beard, no gray hairs, and a traditional Jedi outfit as opposed to his worn down space parka.
Of course, the twist is soon revealed that Luke is not actually on Crait. This is a mental projection Luke is using to distract Kylo Ren so that the few remaining members of The Resistance can get away. But even so, why choose to alter what he looks like? And shouldn't Kylo have noticed that Luke was wielding a lightsaber that had just been destroyed? Writer and director Rian Johnson recently revealed his justifications for the scene to IGN.
First, the saber: "The truth is, we see the lightsaber split in half [but] Kylo sees a blinding flash of light and is knocked unconscious, and then Rey takes the lightsaber away before he wakes up. So if you really want to dig into it and get an explanation, you can say that he doesn’t 100 percent know what happened to the lightsaber." Okay, that feels a little flimsy to me, but also plausible.
As for the haircut and outfit, Johnson's reasoning is a bit more solid. "[Luke] is basically tailoring this projection to have maximum effect on Kylo. He knows that Kylo’s Achilles heel is his rage, and so that’s why he kind of makes himself look younger, the way Kylo would've last seen him in their confrontation at the temple, and that's why he decided to bring Kylo's grandfather's lightsaber down there; the lightsaber that Kylo screamed at Rey, 'That’s mine, that belongs to me'."
I'm sure some who saw the movie already arrived at that conclusion, but in a movie full of half-truths and unanswered questions, it's nice to hear it straight from the man in charge.