Childhood has always been a negotiable concept for Hollywood marketing chiefs.
Right from the earliest days of silent movies, children in films have often been played by young adults while child actors have been given adult traits. In franchise films, in which characters grow up playing the same role, this tendency is exacerbated. There is nothing surprising in the fact that Emma Watson, now 19, is still at Hogwarts Academy in the Harry Potter films.
The cult of youth is one of Hollywood's defining traits, even if it does lead to some strange casting decisions. Film pioneer D W Griffith relished working with very young actresses. There were practical reasons why he was drawn to Mary Pickford (16 on her film debut with Griffith), Blanche Sweet and sisters Lillian and Dorothy Gish (also teenagers when they came into Griffith's orbit). At a time when equipment was cumbersome and performers were caked in make-up and had to work under heavy lights, Griffith's teen stars had a vitality on screen that older performers simply couldn't match.
Directors like Griffith and his silent-era contemporaries – steeped in 19th-century melodrama – also had protective and very sentimental ideas about femininity. They liked to portray their heroines as virginal and innocent waifs. The archetypal Griffith heroine was the character played by Lillian Gish in Broken Blossoms (1919), a young girl adrift in squalid, violent Limehouse. She may be beaten by her father and exposed to every degradation but she never loses her beatific quality. Gish was in her 20s when she played the role but on screen she looked far younger. With her long curly tresses and little hat, she could be mistaken from a distance for Little Bo-Peep.
British cinema of the silent era was likewise dominated by ingenues: actresses like Alma Taylor (who started her screen career at the age of 12), and the equally-youthful Chrissie White, the star of the Tilly, The Tomboy series (1910). "One can watch her with pleasure, irrespective of the part she is playing, because she makes herself felt as an individual and is not merely a puppet: in short, she is quite a little feast in herself," one critic wrote of her performance – an assessment which has a vaguely sinister undertone when one realises that the reviewer is describing a child.
Novelist Graham Greene famously ran into trouble when he reviewed the Shirley Temple vehicle Wee Willie Winkie (1937) in Night and Day magazine. "Infancy with her is a disguise, her appeal is more secret and more adult," Greene suggested of the then-nine-year-old Temple, referring to her "dubious coquetry" and calling her "a complete totsy". Fox sued for libel and Night and Day soon folded. In his provocative way, Greene was pinpointing a disturbing trend: the way filmmakers gave child stars adult traits, risking sexualising them. It was an early example of the Britney Spears syndrome.
What may have been intended as playful comedy about precocious kids wasn't always regarded in such an innocent light by spectators or critics. On the rare occasions that filmmakers acknowledged childhood sexuality and the threat posed to kids by predatory adults, they did so in circumspect fashion. For example, when Stanley Kubrick adapted Vladimir Nabokov's Lolita, he made Lolita (played by Sue Lyon) a teenager, not a 12-year-old as she was in Nabokov's novel.
The rise of the teen movie in postwar America saw the lines between adulthood and childhood blurred more thoroughly than ever before. When you watch James Dean in Rebel Without A Cause (1955) having knife fights, playing "chicken race" in cars and looking moody and misunderstood, it is easy to forget this is actually a film about a boy in high school. Dean, in his early 20s, was playing a 17-year-old. We may see him yelling at his father ("you're tearing me apart") but adults are generally kept at bay. The teens live in a self-enclosed world.
In Griffith's era, youthful actors were recruited for adult roles. In teen movies, the traffic has gone in the opposite direction. Olivia Newton-John was nearly 30 when she starred as Sandy, the demure high school student with the hidden wild streak in Grease (1978). (Her co-star John Travolta was 24.)
It's instructive to compare teen movies like Rebel Without A Cause and Grease with Jonathan Kaplan's Over The Edge (1979), in which the teenage rebels were actually played by teenagers. Matt Dillon was only 15 when he starred in the film. It may not have done well at the box office but it had a roughness and edge that teen movies with older actors "playing young" sometimes lacked.
It wasn't just US high school movies about leather-clad rebels which featured overage actors in the main roles. In The Railway Children (1970), Jenny Agutter, then 18, and Sally Thomsett, 20, were both far older than the Edwardian-era children they were playing. Judy Garland was a mature 16 when she was cast as the young girl Dorothy in The Wizard Of Oz (1939), but she could carry a song with a verve few more-youthful performers could match.
In today's new digital world, where make-up and prosthetics make astonishing physical transformations seem routine, actors can leap between generations as never before. In Jaco van Dormael's forthcoming fantasy Mr Nobody, we'll see Jared Leto playing various different versions of the same character (from a young man to a very, very old one of 120). Emma Watson may not look like a Hogwarts' schoolgirl when she is modelling for Elle magazine or giving interviews discussing her university plans. It's hard, though, to think of anyone else who could play Hermione. After all, when you have 10 years' experience in a role, it's churlish to complain that you're too old to play it.