In the movie “The Mountain” (1956) Spencer Tracy and Robert Wagner portray brothers. To be precise they are portraying step-brothers but since Spencer Tracy is 30 years older than Robert Wagner, that’s really straining believability don’t you think?
Rachel Griffiths played Johnny Depp’s mother in Blow (2001), despite being five years younger than him.
From IMDb:
James Cagney was eleven years older than his screen mother Rosemary DeCamp
In Yankee Doodle Dandy
I dont remember the age spread and I feel to lazy to google it but Glenn Close playing Mel Gibson’s Mom in Hamlet was laughable. IIRC she is les than 10 yrs older than him.
James Dean was 24 when he went to high school in Rebel Without A Cause.If he pursued his unmarried relationship to his film sweetie Natalie Wood he might have been convicted of statuatory rape 
She was 16 at the time.
That’s nothing. Glenn Close is only five years older than Robin Williams, but she played his mother in The World According to Garp.
And back to TV, Beverly Willis plaed Joan’s sister on I Married Joan. Beverly Willis was actually Joan Davis’ daughter, and 24 years younger.
Robert Duvall is only three years younger than June Carter Cash who played his mother in THE APOSTLE.
There was an episode of “Who’s the Boss” in which Mona (Katherine Helmond) gets an unwanted visit from her mother (Angela’s grandmother), who would have to be in her 80s. The actress, Lois de Banzie, was younger than Katherine Helmond and looked it. (She was also supposed to be English but had no accent and was a foot taller than Katherine Helmond; Dame Judith Anderson would have been perfect but…)
Don’t even start with soap opera ages. On DAYS OF OUR LIVES, the character Alice is about 90 but has grown great-great-great-grandchildren.
Bill Cosby is 11 years older than Felicia Rashad, who played his same-age wife on COSBY SHOW. Felicia Rashad was 10 years older than “their” oldest daughter, Sabrina LaBeauf. Cliff’s parents (Earle Hyman & Clarice Taylor) were both 10 when he was born.
Natalie “Lovey” Schafer was 13 years older than her TV husband Jim Backus on GILLIGAN’S ISLAND.
Last post, I promise:
On EIGHT IS ENOUGH Dick Van Patten’s character celebrated his 50th birthday in an episode ca. 1978. Ten years later he celebrated it again on a reunion show and the kids he fathered as a 10 year old came to help him celebrate.
In A VERY BRADY CHRISTMAS (1988) Cindy Brady is still a college freshman in her late 20s, just as she was when last seen in THE BRADY BRIDES got married 8 years before, making this one of the first shows to deal with learning disabilities.
The Waltons Chronology became hopelessly screwed up in the Reunion movies. In a Thanksgiving movie set during the JFK assassination 1963, a baby who was born before Pearl Harbor (1941) was an adolescent while kids born in the 1940s were still elementary age. By the time of the last one (1997), set in the early 1970s, mother Olivia has just become a teacher (even though her character would have to be well over 70 since the series started in the early 1930s and she already had teenaged children) and the Baldwin sisters, who were septugenarians in the 1930s, are still making whiskey even though they’re 115.
Look no further than the classic ‘High Noon’. When it was released in 1952, Gary Cooper was 51. His bride, Grace Kelly was 22.
Michael McCafferty, who played Eberts, the “lifer” assistant to The Official on The Invisible Man was only 22.
Don’t even get me started on the Saved By the Bell kids.
According to the 24 Declassified Season 1 Book, Kim Bauer is supposed to be 14 years old, even though her classmate Janet has a license and owns her own car. Elisha Cuthbert was actually 19 during season 1.
Nicholas Brendon was older (26) than every Buffy cast member but Anthony Stewart Head when he played high schooler Xander Harris. James Marsters (Spike) is currently 41.
Well, in Laurence Olivier’s version of Hamlet, the 41-year-old Olivier played the son of 28-year-old Eileen Herlie…now, I wouldn’t say a word that could be reckoned as injurious, but to find a mother younger than her son is very curious… 
This sort of thing seems to be reasonably common in productions of Hamlet, though not usually so drastic – in the 1980 BBC version, Claire Bloom (Gertrude) was only six years older than Derek Jacobi (Hamlet) – who was two years older than Patrick Stewart, who played Claudius.
Then, too, Maggie Smith played Ian McKellen’s mother in the 1995 Richard III, though she’s only five years older than he is…
Actually, Charisma Carpenter (Cordelia) was the oldest of the “teens,” being a year older than Brendon. And while it is sad that I know this, I’m glad because it made me feel a lot less pervy watching the show after I realized that most of the class were close to my age. Youngest were Tara and Buffy (and Dawn, of course), oldest were Cordelia and Xander.
I looked it up after seeing an episode and thinking, “What the hell happened to Xander? He looks almost older than me!” and then checking the IMDB and seeing that he is older than me.
Tony “Felix” Randall is 83 years old while the actress who plays his wife is only 32, and those kids are wayyy too young to b…
What’s that? She IS an actress but she’s NOT acting?
In episode “Seasons of Belief” from the anthology series “Tales from the Darkside,” 75-year-old E.G. Marshall and 35-ish wife Margaret Klenck tell their small children a horror story about an awful being named “The Grither.”
How about Crispin Glover playing Lea Thompson’s father in Back to the Future?
Glover was born in 1964, Thompson in 1961.
Dammit - I got that one goofed up!
Glover and Thompson played the parents of Michael J, Fox, who was also born in 1961.
John Mahoney is only 15 years older than Kelsey Grammer. They gave him the cane to make him appear more elderly.
Boyz N the Hood
The parents of Cuba Gooding, Jr. (b. 1968) were played by Angela Bassett (b. 1958) and Laurence Fishburne (b. 1961).
Then said book is unadulterated shite.
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Nicholas Brendon was older (26) than every Buffy cast member but Anthony Stewart Head when he played high schooler Xander Harris. James Marsters (Spike) is currently 41. **
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Then surely NB was older than every Buffy cast member other than Tony Head and James Marsters for the last two years? Or are you one of those lame-ass Tara 'shippers who refuses to acknowledge S6 and S7?