Okay, let's get to your questions in some sort of order...
1. Is it acceptable to say "YOUNG SOMENAME" in the dialog headers but only describe them as "Somename" in the actions? What would a non-noob do?
It makes sense
to me that the dialog slugs remain consistent; the description can vary somewhat, since the characters are the same age in the scenes
or is this an unwarranted assumption?2. Is there a formal header that indicates that the characters have grown up or do I just say that in the action? In the action, should/can I put something like "MATURE ANNIE" and "MATURE DARIN" (in all caps) to signal that it's their first appearance in the script as grown-ups?
You might just put in some subtle hints in the description that time has passed; then again, you could always use an on-screen subtitle, "TEN YEARS LATER". And if you use "Young Whatsis" in the early going, just call him "Whatsis" in the dialog slugs.
If it makes you feel better, you can CAP the first appearance of the mature character; I don't believe it's necessary. {But my opinion does not necessarily control.}
3. Is it reasonable to have two different actors play the 14-year old and 24-year old "versions" of the same character in the movie? Or do you think that the same actor should/can pull it off? (I know, this is not really script-related. I just wonder about it whenever I imagine my script being made into a movie).
Since you would only be worrying about this if you were casting it yourself, here are some general guidelines...
There are very few 24-year-olds who can play 14 convincingly.
There are even fewer 14-year-olds who can play 24. {Trust me on this one; I've seen far too many Super8 films where the director, age 14, tries to have his best friend, also 14, play 25... or 55.}
This works about as well as it sounds like it'd work. And I was guilty of that myself... when I was 14.
A 14-year-old who can act, and can
play 14 convincingly, is a treasure greater than gold...
A 24-year-old who can act is a pleasantly surprising find, also.
Two actors are more easily dealt with, anyway... dealing with child labor laws, on-set tutors, child welfare officers... it's something you don't want to have to deal with.
Of course, if it's right for the film, you do it anyway...
Al B.