Basically everything you're saying isn't true. Factually inaccurate. For multiple reasons. First of all, many jobs have age requirements. Many things in this world have different ranges where you can't do it until after a certain age. Perfectly legal. Also, other industries other than movies, including TV, radio, modeling, etc exist and do the same things. Sports and athletics related jobs have gender requirements. So even if we want to pretend that what you're saying isn't BS (it is BS), then it's still wrong because it's not only movies.

Next, OP specifically says the word discriminate. Discrimination is unjust/prejudicial treatment of people based on their given categories. It isn't unjust or prejudiced to pick a person based on their age, race, gender, etc for a role in a film. It's appropriate for the work that is being done. Which is my point. You're the one missing the point trying to have a Reddit "gotcha" moment and you failed.