The labor laws for children (<18) in film are brutal. As they should be. Better to avoid the whole thing unless really necessary. Extras definitely won't be under 18.

Something that a lot of replies are missing is how quickly and dramatically we change appearance at that age. Imagine if filming runs over a year and the 14-year-old you hired is now a foot taller and the shape of his face changed through puberty. Sure, little kids grow fast, but features don’t change nearly as much, especially for boys in their teens.

Directors simply like to work with adult actors more than to deal with teenagers.
Younger actors have less experience.
Teenagers are legally required to take longer breaks and can't work into the night.
And if you're planning on a sequel or a series, you have no idea what your actor will look like in 2-3 years.

Dorothy in the Wizard of Oz is supposed to be 12 years old. Been this way since the day the world turned to color.

I'd bet there's a 'sweet spot' for age where the average person watching a movie can mentally overlook the adult in a teen role, while children and elderly can't be portrayed by a different age without it being a deliberate effect choice or farcical (though when I was learning makeup effects, I saw a ~25 year old turned convincingly into a 60+ person). Maybe it has to do with ease of an adult actor compared to a teen, or maybe it's because there are just more of the 'young' adult actors in the pool than readily accessible teenage actors. Maybe the hiring team wants to ensure they have someone who can act without being taught during the production, and the slightly older actors have more proven track records?

Bo Burnham’s “Eighth Grade” features a 13-year-old playing a 13-year-old. He said he wanted to capture the natural awkwardness and self-consciousness that we all experienced at that age.

zits, probably

Perhaps something related to child labour laws making it more complicated to hire minors?