Chronology problems in this film

Power changes very little in the film. One problem is that he was almost 42 when filming this movie, so having to play a man in his late teens (Duchin was born in 1909, and the film opens at the time of Lindbergh's flight in 1927) was necessarily unconvincing: he looked nothing like someone in his 20s, let alone a teenager. Obviously he looks more "age appropriate" in the latter part of the film as by that point Power was Duchin's actual age (more or less).

James Whitmore was an excellent actor but he didn't "disappear" into the role to the point where his looking precisely the same in the early 50s as in the late 20s is unnoticeable. They didn't even bother to gray his hair, and he was supposed to be older than Duchin, so his failure to age a day is in fact quite glaring -- actually, the most obvious of anyone in the film.

Kim Novak looked fine but as far as aging goes, her character died young, so the entire issue of her being made-up to look older is non-existent.

The movie, too, has absolutely no sense of time or place, or any feel for the passage of decades, other than in the most superficial, unconvincing way.

Sidney was an important director but my comment has nothing to do with whether he was directing programmers or major films. I've usually found him to be a rather heavy-handed, often over-the-top director, lacking the finesse of Vincente Minnelli or the inventiveness of Stanley Donen, to cite two other directors who came out of MGM, as had Sidney. He had some major flops (Pepe being the most notorious), but even his good pictures (and of course he had successes) often could have been better handled by others, or the material was strong enough that he couldn't do it any harm. Some people like Robert Osborne inexplicably think he was terrific but when you look at the body of his work he has a lot of poorly done films as well as some good ones.