- "Sorry for the disguise, Marty, but I was afraid you wouldn't recognize me. I went to the rejuvenation clinic and got an all natural overhaul!"
- "Doc and Marty pushed open the DeLorean's gull-wing doors. Marty climbed out of the car. But when he turned back to Doc, it looked like the inventor was peeling off his face! / 'Excuse the disguise, Marty,' Doc explained mid-peel, 'but I was afraid you wouldn't recognize me. I went to a rejuvenation clinic and got an all-natural overhaul. They took some wrinkles out, did a hair repair, changed the blood — added a good thirty or forty years to my life. They also replaced my spleen and colon.' He pulled the last of the goop from his face and ran a hand through his tangled hair. The hair stayed tangled. 'What do you think?' / Marty had trouble not staring at the new, improved Doc Brown. He didn't look that different, really — but he did look better. Younger. Many of the wrinkles were gone, and there was more of a sparkle in his eyes. / 'You look good, Doc,' Marty answered slowly. 'Real good.' Staring at his slightly dewrinkled friend, his surroundings had really begun to sink in. So this actually was 2015."
- —From Back to the Future Part II by Craig Shaw Gardner (quote, pages 17 and 18)
A rejuvenation clinic was where the physical effects of old age could be reversed, adding "30 or 40 years" to a subject's life and making them appear proportionately younger.
Dr. Emmett Brown underwent the process at either age 65 or 71, some time between his departure from 1985 and his later return, possibly in 2015 though likely even further into the future. Doc thought he looked so youthful that he then added prosthetic age makeup before returning to 1985 so that Marty McFly would be able to recognize him.
Treatments included "hair repair", changing the blood, and replacing the spleen and colon. Hair repair was presumably a way to delay or stop male pattern baldness or reverse it, as for unknown reasons Doc chose to keep his white hair (whether he thought this made him look more distinguished is perhaps best left to personal preference) as opposed to coloring it to blond as it had been when he was thirty years younger in 1955.
Behind the scenes
- In reality, de-aging Doc was simply because the filmmakers did not want to go through the process of making Christopher Lloyd look older anymore, nor was Lloyd keen on wearing any more makeup than was necessary. Plus, they didn't want Doc to be physically in his seventies when entering a relationship with Clara Clayton in the third film, or while going through rigorous physical events of the second and third films — such as jumping between the cars of a speeding locomotive, for example.
- Since there isn't anything close to a rejuvenation clinic in medical technology, it's possible that Doc went even further in the future. Having access to a time machine, Doc may well have made some investments in the past in order to afford what would likely be an astronomical fee for such an overhaul, or traveled into the future to a date when such services were routine.
- Even within the Back to the Future universe, there is no indication that major medical procedures like organ replacement were common as of 2015, nor that new organs were available as part of rejuvenation (transplanted organs, from donors, had been available in 1985). Although there had been advancements in plastic surgery, and businesses like Bottoms Up that provided the service, the type of wrinkle removal that Doc had does not appear to have been available; the elderly people seen by Marty in 2015 — Biff, Terry, George and Lorraine — look their ages. In addition, after teenage Jennifer was picked up by Officers Foley and Reese, they were surprised to hear that, according to their identa-pad, she was 47 years old, with Foley commenting "47? That's a hell of a good facelift."
- Finally, Doc simply says that he "went to the rejuvenation clinic", though not necessarily in 2015. The only information that Doc needs to impart to Marty is that (a) Marty needed to be in 2015 to prevent Marty Jr. from committing a crime and (b) there is a reason why Doc looks younger than he did when Marty had seen him last.