So many! If I recall the majority of original BH90210 cast was mid-late 20's when the show started. Both Tammin Sursock and Bianca Lawson (PLL) are in their 30's. Bianca is like 36 and has been playing a teenager mostly for the last 20 years lol
it's rare to find an actual teenager in teenage roles these days (aside from disney channel stuff).
Elle
"Strangers have the best candy"

Shirley Henderson played Moaning Myrtle in the Harry Potter films; she was 35 years old when she first appeared as a teenage ghost. Only by freezing the screen can you tell she is old - her lines show a bit but otherwise you wouldn't notice

It is normal for actors in their late 20s to play teens
I must become someone else. I must become something else.

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Bianca Lawson - 31 when she played a high school student on PLL

Shirley Henderson was 40 when she played Moaning Myrtle in Harry Potter and the Goblet of Fire.
This will be the high point of my day; it's all downhill from here.

Most of the cast of the tv series Popular was at least ten years older than what they played. Most of them.

Let's not forget Eric Christian Olsen. He plays a high school character in the new movie
Fired Up
and he is 32.
The mark of my Deity shall scar thy DNA.

yup, and she looked really really young!
"and the amazing thing: still more romantic than Faith." Xander

Stacey Dash was 29 when she played a 15/16 year old on Clueless

Jason Earles plays a 17 year old on
Hannah Montana
but though there arebacked uprumors that he is actually 31.
Synetech

oh my gosh she looked like 15 years old on dawons creek! they did a great job on making it look like she was actually younger than what she was!

Stockard Channing also who played Rizzo in "Grease" she was in her 30's when she did that role. There are most probably many others who won't tell as sometimes it can work against you.
She did look that old though! Meredith looked young in DC and still looks younger than her age now - which probably works against her for more grown up parts! Can't win!

If you've ever seen the low budget film Serial Killing 101. Lisa Loeb (the singer) played a high school student and she was like 35 or 36.
I only watched cause I'm a fan of hers. It was actually funny in a few parts.
I find your lack of faith disturbing.

Erica Hubbard is 29 years old and she plays a convincing 16 year old on Lincoln Heights.
Shoot for the moon, and if you miss you are still amongst the stars

Stockard Channing also who played Rizzo in "Grease" she was in her 30's when she did that role. There are most probably many others who won't tell as sometimes it can work against you.
I've only just discovered Meredith's age today, but wow she surely proves that age is nothing but a number as she surely looked nowhere near her age in "Dawson's Creek". In fact she looked better than some of the cast who were younger than her at the time.

Gabrielle Carteris in Beverly Hills 90210 was 29years too.