Do actors/actresses aged 20-21 often pull of the illusion on film of being 17?

I ask this for the sake of Dan Radcliffe, Rupert Grint, Emma Watson (and the rest) and the final film in the series.
edit:And also for Dakota-Blue-richards who has got to be between 11 and 12 for the whole HDM franchise (assuming the next two films happen) while currently being 13 IRL (and therefore probably being 15 or 16 by the time of the last film).

edit 2: (judging by what I’ve heard: Her actual age does not seem to be that important)

I reckon that by the time that film is being made those people will be around 20-21.

I know that some people can do it (MJ Fox played a teenager in his thirties I believe, in BTTF 1, 2, and 3. But he was a special case)

What examples are there of actors over 20 playing characters in their teens (and being believable)?

‘off’ not ‘of’ damn fool!

Is Ralph Macchio the Karate Man yet?

Summer Glau, who plays Cameron in The Sarah Connor Chronicles, is 26. She plays a cyborg that’s supposed to pass for a 16 year old (or thereabouts) human girl. She at least looks like a teen-age girl, IMO.

It doesn’t seen all that difficult to me. I mean, I worked with an 18-year-old girl recently who could have passed for 12. Were she an actress, I could see her portraying teenagers well into her 20s, and maybe past 30.

Alison Lohman is approaching 30 and has never played a character who was supposed to be over 18.

Good point. I was very surprised to realize that Summer is about a year older than Jewel Staite, because in ‘Firefly’ they were very good at creating a dynamic in which Kaylee is the big sister and River is the little sister.

In the recent hit “Juno,” actress Ellen Page, who is 20 IIRC, plays a girl of 16 in the film. I didn’t have any trouble buying it.

I thought using adults to play teenagers was fairly standard. It gets you around all sort off laws about minors working, and (this is big) you don’t have to deal with teenagers flaking out, having growth spurts and generally being a PITA.

Matthew Broderick, Jennifer Grey and Alan Ruck were 25, 26 and 30 respectively when their movie Ferris Bueller’s Day Off was released. I totally bought them as high school students. Some people just look young for their age, and Hollywood gets first pick of them.

And then the Valley gets 'em.

Ew! Well, probably, yeah.

She also played a girl who was about 13 or 14 (?) in Hard Candy and managed it very well.

I do think the guys in Harry Potter are starting to look a little long in the tooth for high school. I’m finding them less and less believable as young kids, even though I’ve worked with high school kids and know full well that some of them can look like grown adults (and still act like jackasses).

Some of the Buffy actors (in the early years) were in their mid to late 20s, but played fairly convincing teens. Nick Brendon seasons one and two and Alison Hannigan in the first four seasons especially. Hell, Tom Lenk (the guy who played Andrew) is currently in his early thirties, but still looks about sixteen.

Of course, the later years (especially for Nick B.) were a different story. But the Harry Potter kids probably wont have any trouble.

She doesn’t look that young. She has the looks really young body of a dancer/ice skater/gymnast (I think she is a dancer), but you can see her age in her face.

In Buffy, Xander was 26 and Cordy was 27 in the first season.

Matthew Broderick you could sort of buy as a high school student, but Alan Ruck looked like he was ten years older.

Maybe it’s just me, but the Summar Glaus and Alyson Hannigans of the world don’t fool me for an instant, not even long enough to suspend disbelief. Getting little Ellen Page, 20, to play someone 16 isn’t a stretch because four years just isn’t a lot of difference anyway. But Summer Glau doesn’t look like a teenager, she looks liks a woman in her late 20s. (Looking it up on IMDB, she’s 27.) It’s her eyes; there’s something about a person’s eyes, about the way the face moves. Shows like “Beverly Hills 90210” and “Buffy the Vampire Slayer” were just absurd to me beyond belief, watching these 25- and 30-year-olds pretend to be teenagers.

I can think of only a handful of actors who’ve pulled this off to my satisfaction, and most of the time it looks ridiculous.

In the last film, which will most likely start shooting late this year or early next, they will be playing age 17, and their actual ages will be 19 (Dan), 20 (Rupert), and 18 (Emma). This is well within a minimal difference where you wouldn’t be able to determine their true ages from their appearance. I anticipate no problem with it, and really it’s the continuity of using the same actors every time that will matter more; an almost unprecedented feat for such a long series.

I have no problem with actors playing younger, or older, if it’s under 10 years difference. If they are lucky enough to have that ageless Michael J Fox/Ralph Macchio look, then I’d stretch that to 15 years.

“Dawson’s Creek” featured a lot of older people playing younger. Mostly, they didn’t convince me they were really fifteen…but Meredith Monroe, who was born in 1968 was actually very convincing, considering how much older she was. When she came on the show in about '99, she was in her early 30s, playing a sixteen year old. I didn’t think she was sixteen, but I assumed she was no older than Michelle Williams or Katie Holmes…so, some people really do have that young look, I suppose.