Age might not be anything but a number, according to R&B singer Aaliyah, but it is important in establishing realism.
Still, a lot of recent casting announcements seem to be placing familiar actors in roles that don't seem age-appropriate.
Sure, as audiences, we can suspend our disbelief, but that willingness may be stretched to the breaking point in some of these upcoming films.
Here are a few of them, from the "How Will This Work?" file.
Helena Bonham Carter as Miss Havisham
Earlier this week came the confirmation of the long-rumored casting of Helena Bonham Carter in an upcoming remake of Dickens' 'Great Expectations.' When I read about this, I wondered what her role would be, since she's too old to play Estella and too young to play Miss Havisham.
But in fact, the 44-year-old will be playing the latter role, the batty old dowager who's never taken off her wedding dress since being left at the altar decades ago.
Bonham Carter can certainly bring the crazy, but she hardly seems mature and wizened enough. (In the last major remake of the story, Anne Bancroft, then 67, played the role.)
Gemma Arterton and Saoirse Ronan as mother and daughter
Even more curious was the announcement of 'Byzantium,' director Neil Jordan's return to vampire movies, which will star Gemma Arterton and Saoirse Ronan as vampires who pose as sisters. That's believable, but they duo is actually supposed to be mother and daughter.
This is problematic because Arterton is 25 and Ronan is 17. So Arterton was 8 when she gave birth? Okay, vampires don't exactly age like we poor mortals do, but still, does that make sense?
Leo DiCaprio and Carey Mulligan in 'The Great Gatsby'
Then there's Baz Lurhmann's upcoming version of 'The Great Gatsby,' starring Leonardo DiCaprio as Gatsby and Carey Mulligan as Daisy. Fitzgerald fans know that Daisy is supposed to be Gatsby's long-lost love, which suggests that they're more or less the same age. But DiCaprio is 36, and Mulligan, who was playing teenagers just a couple years ago in movies like 'An Education,' is 25. Did they have their long-ago romance when she was a child?
Kiefer Sutherland and Kate Bosworth as a couple
Some other eyebrow-raisers: The forthcoming 'Fairytale of New York' will star Kiefer Sutherland as an Irish comedian who falls for an American woman, played by Kate Bosworth. The producer says the two stars have a "magical connection," which is good because Sutherland is 44 and Bosworth is 28. Actually, that 16-year age gap is modest compared with some of the romantic pairings the movies try to sell us.
Joseph Gordon-Levitt and Bruce Willis as the same person!
Also intriguing is 'Looper,' a sci-fi thriller picked up for distribution this week starring Joseph Gordon-Levitt as a time-traveling hitman who discovers that one of his targets is a version of himself, 30 years older (Bruce Willis). Gordon-Levitt is 30, Willis is 55. Compared to some of these other movies, this time-twisting plot and casting actually make sense.
•Follow Gary Susman on Twitter @garysusman.