Why this practice is so frequent?
Without being inside the minds of casting directors, part of it may be because of labor laws that apply to child actors. If the actor is under a certain age then there are limits as to how many hours a day he or she may work. That’s why babies and very young children are often played by twins. There may also be required tutors on-set and other regulations which increase the overall cost of production.
Not to mention that with adolescent actors come adolescent woes: pimples, weight gain (and loss), voices cracking at the wrong time, clumsiness on set and on camera. If I was the wardrobe mistress of real teenagers, I’d have to build or buy three of every outfit, because the damn kids grow so fast. Twenty-somethings are past all that and as pretty as they’ll ever be in their lives (in most cases).
But Otto’s explanation is the one I’ve heard most often.
Another WAG: in many cases, they’ve actually had a little more time to study acting.
Luke Perry is 40.
It’s mainly a US thing, AFAIK- UK/Australian shows tend to use teenage actors/actresses for High School Soaps and Dramas.
The problem is that, even with make up and so on, a 15 or 16yo actor/actress still looks 15 or 16… and that could make viewers (and the actor/actresses’ parents!) uncomfortable with some of the events/situation in the storylines (especially anything involving drugs or sex).
Also, teenagers aren’t exactly noted for their maturity (for the most part), and there’s the Real-Life dramas they could very well bring to the set (“She tried to pash my boyfriend!”/“He stole my PlayStation game!”/“S/he is a lezzie/gay!”)
You get the idea. Probably easier just to use early twentysomethings and assume no-one will notice or, more importantly, care. 
Lack of maturity, coupled with self-absorption and self-importance?
Source: http://www.cnn.com/2006/SHOWBIZ/Movies/07/29/lohan.ap/index.html
Perhaps older actors have had previous work in the industry, making them a safer and more reliable choice (and cheaper to find) than auditioning a bunch of unknown teenagers the correct age for the role.
Also, movies tend to deal with fairly long-term time frames: it can take a year between the initial screenplay to the final release. Once a film is cast it might be two months before the cast is assembled for a 6-12 week shoot. A teenager could potentially change quite a lot in that time period. He might not look much like the headshot his agent submitted.
Regarding the voice-cracking thing: lines in movies can be looped in later in ADR (additional dialogue recording). ADR helps quite a bit, especially when there’s a lot of foley work (sound effects, shoes squeaking, doors closing, ambient noise). It’s probably easier to make the Spanish-language version of your movie if the sounds are already pre-separated on the English-language version. ADR is usually an unnecessary expense for a TV show — first, you can get the sound right on-set, and second, TV is usually local, so you don’t have to translate it into eight languages for the international release.
There’s also the problem that if the series is successful, you end up with an actor that is five years older, but the producers may have kept the characters in high school to keep the plots and situations. Buffy is a bit unusual in that the characters were allowed to go to college, although I think they cheated a little bit.
According to IMDB, Gabrielle Carteris (Andrea) was 34 when she left 90210 in 1995. An entire high school career older than Luke.
I have to agree with Martini Enfield. Much of the fan base for shows like the OC is actually much older than the high school age of the characters - college students and 20 somethings. I don’t think they are interested in watching characters that still look like children.
If you’ve ever worked with teenagers you’d know the answer to this question.
Not sure what you mean by cheated, but of the original main high school student characters, Buffy was the only one who was anywhere near her character’s age. The first season was their second year of high school, so the characters were around 15 or 16. Actual ages:
Buffy: 20
Willow: 23
Xander: 26
Cordelia: 27.
Or to use a more recent example, Veronica Mars where the characters were supposed to be high school juniors in the opening season (2004). Their actual ages were:
Kirsten Bell (Veronica) - 24
Percy Daggs (Wallace) - 22
Jason Dohring (Logan) - 22
Francis Capra (Weevil) - 21
Teddy Dunn (Duncan) steadfastly refuses to reveal his age. But his actual high school graduation was in 1999 so figure he was around 23.
Just in case you think this is a new practice:
When The Many Loves of Dobie Gillis started in 1959, the characters were juniors in high school. They were played by:
Dwayne Hickman - 25
Bob Denver - 24
Warren Beatty - 22
Tuesday Weld - 16
Weld was soon kicked off for bad behavior (take that Lindsey Lohan) and Beatty didn’t like his role. They were replaced by Sheila James - 18, and Stephen Franklin - 27.
And Hickman had just spent five years as a teenager on The Bob Cummings Show, making him one of the few actors to play a teenager for a full decade after leaving his teens.
Expano Mapcase
Small nitpick.
That actor’s name is Stephen Franken and not Franklin.
Well on Buffy they had to stop referring to the characters ages because it would have just been ridiculous. They looked old by season 7.
I always thought it sucks that TV shows do that because your average teenager has enough hangups about their looks without having to see model-perfect 25 year olds as the standard their looks have to measure up to.
One unexpected source of realism was the cast of Saved by the Bell (ages in the first season of 1989):
Mark-Paul Gosselaar (Zack) 15
Mario López (AC) 16
Dustin Diamond (Screech) 13
Lark Voorhies (Lisa) 15
Tiffani-Amber Thiessen (Kelly) 15
Elizabeth Berkley (Jessie) 17
Imagine that - a show about teenagers with an actual cast of teenagers.
Mila Kunis played 16 yr-old Jackie Burkhart when she was only 14! When she auditioned the casting people asked how old she was; she responded “I’ll be 18 on my birthday.” She just didn’t say which birthday.
Isn’t the actress who plays Moaning Myrtle in the HP movies in her 40’s?