In the cheesy pre-MCU Marvel movie Ghost Rider, starring Nicolas Case, it started out in the past when his character and his love interest were childhood sweethearts, both around 15 years old. Then they grew up to present day, being played by Cage and Eva Mendes. I remember a snarky review saying “somehow Nicolas Cage’s character aged 10 years faster than Eva Mendes’ character”. 
In one of the films in Cocteau’s Orhpic trilogy, he had Jean Marais play a school boy. In short pants, yet. It was really ludicrous.
True. This case just seemed a little odd to me, because there didn’t seem to be any reason to have the character be anything other than actor’s age - but now that I realize that a firefighter of Lowe’s age would be at retirement age, there’s a good reason for the difference.
Found it!
It’s the movie The Man Who Shot Liberty Valance, from 1962.
Irish actor O.Z. Whitehead not only played a teenager, but a teenager who is “too young to vote”, at the tender age of 51. He’s standing in the middle.
Exactly the same thing happened in Kiss Kiss Bang Bang (2005), where Robert Downey Junior and Michelle Monaghan were supposed to be childhood friends, despite the fact that he was 11 years older than her, and looked 20 years older.
I still love that movie though, so I don’t care.
Val Kilmer was great!
It’s a fairly well-known bit of trivia that in Chitty Chitty Bang Bang, Lionel Jeffries played Dick Van Dyke’s father, despite being six months younger than Van Dyke. Jeffries was 42 years old at the time, playing a character primarily known as “Grandpa.”
Jeffries was prematurely bald, and so spent most of his career playing characters who were decades older than he was.
Abe Vigoda was only 50 when he filmed The Godfather. He is another actor who was prematurely old.
In The Manchurian Candidate , Angela Lansbury (37) played the mother of Laurence Harvey (35).
Who, of course, also played a boy who still had all of his baby teeth. 