Christopher Lloyd would have been 47 in 1985, yes, but Doc Brown would have been in his 70s by then. All the characters who appeared in both 1955 and 1985 – George, Lorraine, Strickland, Doc, and Biff – were played by actors appropriate for their 1955 ages and then aged up (well except maybe James Tolkan as Strickland) for their 1985 scenes.
In the sequel, one of the first things 77-year-old Doc does is strip off his aging mask (which Doc had used to hide his de-aging treatments from Marty) so that Lloyd could play him without makeup.
Powers &8^]