I just watched this movie on dvd and I think the casting was not very good.
Firstly, the main character is a little too old. Shouldn't they cast someone in their thirties or forties? Instead they picked someone more than 50 years old.
Then there is Kitty. Why would an 18-year old, good-looking girl want to marry a 50-year old man? That is just unbelievable.
It's called "acting" and "using common sense to suspend disbelief."
Colman's role eventually ages into his character being perhaps a few older than himself. Most actors and actresses play characters older and younger than themselves. Respectfully advise you to either get over it or switch to reading books.
And remember, Colman's character had endured the kind of wartime horror and trauma that would age anyone.
Now, where do you get that the Greer Garson character starts in the story at "18?" Do you really think that an 18 year old girl could be the star attraction of a British music hall?