Some older actors playing kids really look young enough to pass for the intended age: in other cases you just have to go with it.
Leonardo DiCaprio: DiCaprio has always looked younger than he is, which comes in handy sometimes, as when he played con man Frank Abagnale Jr in Catch Me If You Can. DiCaprio was 28 playing a 17-year-old.
Patty Duke: The actress was 15 when she played the (in real life) six-year-old blind and deaf Helen Keller in the film version of The Miracle Worker (1962), winning an Oscar for best supporting actress. She had played the physically demanding role on Broadway from 1959 to 1961.
Isabelle Fuhrmann: Spoiler alert! In Orphan (2009), Fuhrman plays Esther, a mysterious nine-year-old orphan adopted by a couple whose marriage is strained after the loss of a child. Esther is something of a surprise: not only is she an insane multiple murderer, she is also a 33-year-old with stunted physical growth. Fuhrman was 12 when the film was released, so she was playing both older and younger than her real age.
Shirley Henderson: When she played Moaning Myrtle in Harry Potter and the Chamber of Secrets, Henderson was 37 - her secondary-school character would have to have been at least 20 years younger. Given her character is a ghost who died half a century earlier, maybe we can assume the afterlife has its own rules on ageing. She played the role again when she was 40.
Gabourey Sidibe: Making her film debut in Precious, based on the novel Push by Sapphire, Sidibe was 26 playing a 16-year-old.
Wood Harris: In Remember the Titans, the high school football team is the usual gang of geriatrics Harris was 31 at the time. Ryan Hurst and Ethan Suplee were 24 and Donald Faison was 26.
Brad Pitt: Perhaps this is cheating a bit.Pitt in the title role of The Curious Case of Benjamin Button played a character who aged backwards. Makeup, CGI and motion capture were used to create this illusion. Sometimes talent just isn't enough.
Matthew Broderick: The youthful-looking actor played the title character in Ferris Bueller's Day Off when he was 24. Alan Ruck, who played Cameron, was 30 but Mia Sara was in fact a teenager, at 19.
Michael J. Fox: Also younger looking than his years, Fox played teenager Marty McFly in Back to the Future an the title role in Teen Wolf (both 1985) when he was 24. He was nearly 30 when the third Back to the Future film was released in 1990.