Stewart looks about 59 for our 2020 standards.
Or even older.
Certainly he looks like a mature man just about to turn over the hill, not like a barely-not-young guy.
These were worse-
https://www.imdb.com/title/tt0056217/trivia/?ref_=tt_ql_trv
"Several reasons have been put forward for the film being in black and white. John Ford once claimed it added to the tension, but others involved with the production said Paramount was cutting costs, which was why the film was shot on sound stages at the studio. Without the budget restraints, Ford would have been in Monument Valley using Technicolor stock. It has also been suggested that since both John Wayne and James Stewart were playing characters 30 years younger than their actual age (Wayne was 54 when the movie was filmed in the autumn of 1961 and Stewart was 53), the movie needed to be in black and white because they would never have gotten away with it in color. The age difference was particularly noticeable in Stewart's case, since he was playing a young lawyer who had only just graduated from law school and had moved west without even practicing law back east."
He was also way too old to portray Charles Lindbergh in The Spirit of St. Louis-
https://www.imdb.com/title/tt0051003/?ref_=fn_al_tt_1
He did not want to take this role as he thought he was too old. He was 47 and played a character that was 20-25.
Studio execs convinced him to take the part because he was a star, bankable, 6'3" and slim like Lindbergh, and bore a passing resemblance to the aviator. Stewart wanted to portray Lindbergh in a film in the mid to late thirties when he was the right age but the project was shelved.
He should have turned down both of these roles.
Rear Window? A tad long in the tooth but not that bad imo. A good Hitchcock movie.
RW is excellent.
His interpretation of a 45 yo, science fictional.