(notice that there are no grandparents appearing in the movie)
But Uncle Charlie gives Emma a picture of their parents from 1888, which Ann or Roger notes was 53 years earlier.

I always thought McDonalds looked about right for his age. Back then at least. He was almost 30 and I was guessing his character was supposed to be around 30. T. Wright's character I'm guessing was 19 or 20.
"Life is like a box of Krispy Kreme donuts".

Well, let's take a look at the ages of the actors when the movie was recorded. The movie is dated to 1943, but I will assume that it was recorded in the year before (which I think is standard), that is, 1942.
Then:
Young Charlie - Teresa Wright, b. 1918, age 24
Uncle Charlie - Joseph Cotten, b. 1905, age 37
Jack Graham - Macdonald Carey, b. 1913, age 29
Joseph Newton - Henry Travers, b. 1874, age 68
Emma Newton - Patricia Collinge, b. 1892, age 50
Herbie Hawkins - Hume Cronyn, b. 1911, age 31
Fred Saunders - Wallace Ford, b. 1898, age 44
Ann Newton - Edna May Wonacott, b. 1932, age 10
Roger Newton - Charles Bates, b. 1935, age 7
My comments: Henry Travers was way too old to play the father of so young children (Incidentally, he had the same life span as Winston Churchill, 1874-1965, and he was English too!).
Patricia Collinge was perhaps also a bit too old to play the mother, but this was in no way extreme. Contrary to what was written in another thread, she probably didn't get married very young, since her oldest child is supposed to be only 18. Maybe she was about to become an old spinster, perhaps taking care of sick mother or so (notice that there are no grandparents appearing in the movie) but finally succeeded to find a husband, albeit one much older than her.
Macdonald Carey certainly looks much older than he was, just five years older than Teresa Wright. Although her character Little Charlie was supposed to be only 18, the agegap is not so extreme, it just looks so because he looks so old. But maybe Jack Graham was supposed to be older, considering his advanced position within the Police (or FBI).
Hume Cronyn was very young to play the best friend of such an old man, he was 37 years younger than Henry Travers.
Although Emma was supposed to be the older sister of Uncle Charlie, I think that the agegap was supposed to be smaller than 13 years.

Well, Charlie is about late teens or early 20s. So I'm guessing the parents are supposed to be late 40s/early 50s. Which now is young, but back then is not. People didn't seem as young then.

I think Hitchcock liked the actress who played the mother.
But, it is possible that the two younger children were surprises.
"Two more swords and I'll be Queen of the Monkey People." Roseanne

A man can become a father at any age. But the woman was way too old to be the mother of the two youngest.
Margie

People who were the generation of Young Charlie's parent would have lived through some really horrible times. The Depression and then the war, I'm not shocked if it aged them a bit.

I'm a huge classic movie buff and one thing I notice..the actors ALWAYS seemed old for the supposed ages of the characters. And parents who seemed late 50s having teenaged and pre-teen kids was normal.