Despite this trope being a prime source for ridicule, mockery, and parody, there are actually a lot of legitimate reasons for why this is done.
# Labor laws and regulations apply to acting just as they do any other profession. There are a whole string of laws that child actors are subject to, including (in the UK and the USA) education requirements and restrictions on night shoots and work hours. These aren't (always) ideal conditions when trying to meet the deadlines for finishing a film or a TV season.
# On a similar note, several countries require a parent or legal guardian to be present whenever a child actor is on the set. It's challenging enough arranging for a child to take days or weeks off school to appear in a film or television program; asking their parent to give up the same amount of time as well is doubly complicated.
# The vast majority of professional actors are twenty and up anyway, so if you hold an audition for a teen role, nine out of ten of the actors you see will be an inappropriate age, and they'll be the ones with good [=CVs=].
# The pressures of stardom are harder for a teen actor to handle than a young adult. An actor in his or her twenties is less likely to leave a show after one season (or film series after one entry) to focus on his or her education than an actual teenager is.
# It's easier to see the difference between a teen and a young adult if you are a teen or young adult. Casting directors, on the other hand, are often middle-aged or older. To them, a 16-year-old ''doesn't'' look noticeably younger than a 21-year-old.
# Puberty tends to be extremely fickle. A cute 14-year-old can transform into a gawky, gangly 16-year-old with rather shocking celerity, while a cute, 13-year-old GirlNextDoor type can [[SheIsAllGrownUp grow up into a sex bomb]] by the time she's 18 -- if not [[JailBait sooner]] (which, as detailed below, creates [[PaedoHunt all sorts of other issues]]). In addition, most movies and TV shows are not shot in the order they are in on screen, so you could have someone leave a building as a cute little tyke, appear outside looking almost teenage, then become cute again when they reenter the building. Of course, this won't happen if you've already been through puberty.
# Television series tend to be filmed over the course of several months or even ''years''. Because of this a child actor can run the risk of outgrowing the age of their characters, so television shows cast older actors who wont age as noticeably. The age difference between a 15 year old and an 18 year old is extremely noticeable. [[NotAllowedToGrowOld The age difference between a 20 year old and a 23 year old however isn't.]]
# While voice-acting is less of an issue, voices still break without much warning, complicating recording sessions even if the actor is capable of emulating their younger voice afterwards. Professional child-voice actors don't have these difficulties, and usually have a wider range anyway.
# [[PaedoHunt Obscenity and child porn laws.]] In the US, it's generally illegal to do anything that even simulates sex on film[[note]]And in many countries, even in animation.[[/note]] unless everyone involved is 18 or older. Therefore, having older actors allows the filmmakers to depict various acts on camera that would get them thrown in jail if the actors were underage.
# Related to the above, this allows for {{fanservice}} without running into complications ... laws against child pornography generally forbid actual child nudity, of course, but they often also forbid "sexualized depictions" of children, which is just enough of a wild card to make filmmakers nervous: is a panty shot a "sexualized depiction"?
# Most successful teen actors -- i.e., the ones with the most experience and impressive resumes -- [[TeensAreShort tend to be physically smaller than other actors]], and look younger than their actual age, because they are the child-analogues of Dawson Casting (due to more experience, more able to remember lines, less likely to go completely OffTheRails [[CloudCuckooLander on some random whim]], etc.), better-suited to playing younger children than real children would be.
# [[StageMom Stage parents]] can be a pain in the ass for filmmakers, butting in and [[WagTheDirector trying to dictate]] various parts of the production.
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This trope is less noticeable with females as it is with males as females tend to fully mature physically at an earlier age, usually by 14 or 15. Males, on the other hand, tend to not fully mature until their late teens (usually between 17 and 19). Which makes a man in his 20s (or kid in his late teens, for that matter) playing a 15-year-old less believable than it is with a woman of the same age playing a 15-year-old. Further making it worse is the fact that most shows focus on the actor's faces. A woman can keep very similar facial features even after going through puberty, whereas the deepened voice and the stubble on a guy's face (even if a guy shaves, it's still faintly noticeable) make the age gap much more noticeable. For example, look at Austin St. John from ''Series/MightyMorphinPowerRangers'', at age 18, playing a high-school freshman. [[http://a3.ec-images.myspacecdn.com/images01/31/63039052fc135e5f1bed3aa4349a0ace/l.jpg It doesn't look right.]] Now look at Creator/AmyJoJohnson, older than Austin at age 22, playing a character on the same show, also a high school freshman. [[http://images4.fanpop.com/image/photos/22800000/amy-jo-johson-amy-jo-johnson-22845809-1024-768.jpg It's noticeably less obvious.]]