Dawson creek dating
If anything finding a 19 year old to play a 17 year old is well ahead of the curve.
Compare Age Lift, Age-Inappropriate Dress, and Three-Month-Old Newborn.
By and large this trope requires some research, you can't know for certain that an actor/actress is a certain age unless you scour around Wikipedia or IMDb.
Also consider that production time may vary greatly from release of the movie or airing of the episode. Timeline issues sometimes make this more apparent, as 5 seasons of a tv show may only cover one year within the context of the story, making an actor who was an aversion become a straight example over time.
If younger, casting usually tries for children actors, and if older it is simply far less noticeable.
Hard to Get: In the chapter entitled "Catwalking & Starf**king" Robertson remembers her days and nights frolicking around Hollywood with hot actors like Adrian Grenier.
According to the book, they met at an A-List mansion party; she writes that they bonded over a "mutual loneliness in LA," yet she didn't feel a spark.
In other cases, some stories work with this because a key plot point may be the character does not actually look their age and are trying to pass off as older than they really are.
But with no such luck Hollywood will get by with appropriate clothing, hairstyles, makeup and mannerisms.
Consider also that the exact age of the may also be in question (a generic "high schooler" can be anywhere from 14 to 19).