American society has an interest in looking young and staying young and the Entertainment industry actually promotes this idea. But not necessarily just because of culture, it is because the industry has unique applications of labor laws that affect film and television shoots.
The biggest case of this is a term in the industry called “18-to-look-younger” (abbreviated: 18tly). This is when a production uses someone who is at least 18 years old to portray someone younger; sometimes this is known as a “Disney Kid,” since Disney live productions have a habit of doing this. Usually, this is for scenes where productions need high school kids and using an 18-year-old serves a few benefits:
As an 18-year-old, they are a legal adult and therefore, you don’t need the clearance of an adult’s permission.
An adult can work longer hours, including overtime. Minors are not permitted to work overtime. This is useful in a production that may normally shoot 12 hours.
And adults, if non-union, can be paid less than a child so it is cheaper for production. Usually, a child on set is paid similarly to a union actor. This also discourages constant uses of minors on set unless it is necessary for production.
Many productions do have shows and movies set in high school so for many actors, their interest in staying and looking young is not so much about social pressure to stay young (it plays a part in it, though), as it is to land more work. So occasionally, I see the awkward moment when someone who is 36 trying to get away with insisting that they are young enough to look like they are in high school.
Then comes the other question that people ask about age when it comes to age-specific casting: “Isn’t it age discrimination to not use someone who is 40 to play a high school student?”
The answer is “yes.” If a person is 40 and actually looks like a high school student, you cannot use age to bar him from work. That is actually illegal as age-discrimination.
However, if someone looked like Christopher Walken and wanted to be a high school student in a movie, casting can say that he does not look the part because it is not a realistic look or it is not what production is looking for. His appearance would be a condition of work, not the age.
This is a rule of thumb for any casting in a production: Whenever they mention “age,” they mean “apparent age” not “actual age.”