They can’t seem to get it right

*trips and falls in pokéball* Oops! You caught me

I think this was discussed once about why we see college age or older acting as high school age. The gist was that good young actors are harder to come by and actors at college age and beyond tend to have more skill. So to fill parts it's just plain easier to get a bunch of okay college actors since their skill level is about the same as a really good younger actor. It can be done, all kinds of shows use actual kid actors but if you have an easier and possibly cheaper method producers will go for it.