No kidding. Before Lucas clarified the chronology in 2001 (on the TPM DVD) we honestly didn't know how much time had "officially" passed between the time we see him as a child and the time he takes his mask off.
Before the prequels were made, it would have been reasonable to assume a 50 year gap (say, Anakin was a 20-something year old Jedi when the Empire was formed and he became Darth Vader, then about 50 years of Imperial rule before he kills the Emperor, that's what a lot of us fans imagined). So it's Lucas who's created this discrepency, so we're left to assume that somehow he appeared to age faster than normal somehow (much like the Lars family).
The trouble is, Obi-Wan is about the right age for where he is in the OT (only about 7 years off of Sir Alec Guinness) and Palpatine the same (he's in his 80's in the OT if we accept the 50 year old Senator in TPM).
I don't even care that this creates a slight discrepency with the EU, wherein we have a 60-something Boba Fett still jetting around looking for bounties and fighting (as far as I'm concerned he died on Tatooine, but I guess he's just really remarkable physically for his age).
My crack about the 70's thing was just to point out that actor Sebastian Shaw, who plays Vader unmasked in ROTJ is still shown when Luke takes the mask off. He's a 70-something year old man. Then his "ghost" appears and he's 21 year old Hayden. Originally he just appeared as himself, just with his hair back and without all the scarring and facial injuries (and we presume his spiritual body mimics how his earthly body would have looked restored at his age).
If Lucas just wanted to show Anakin "healed" he should have taken Hayden and put him into "middle age" makeup to look like a dignified man in his 40's. Instead it's clear he just decided to make him look like Anakin did right before he officially turned to the Dark Side in Episode III, complete with the hairstyle he had back then, etc (only difference is he has his right forearm back and no evidence of that facial scar, though it's hard to see exactly). So Obi-Wan and Yoda just appear how they looked at their deaths, since neither turned to the Dark Side. Lucas's own explanation on the ROTJ DVD is rather vague, but I'm guessing this is what he meant by it, now having seen Episode III.