I agree with Sherman Logan: a character can be fully developed - in the sense that the audience (or reader) knows what he is like, his values, his probable responses to a situation - without needing “to develop” or change significantly in the course of a work of narrative art.
Look at some of your great movie characters: General Patton, John Wayne as (fill in character ;-), Major Whittlesey in “The Lost Battalion.” They are fully realized characters in their context, which is a combat-based context just as LOTR is. They don’t need growth because they have grown already.
A character doesn’t need to “grow up” to grow. A character needs to experience change during the story. He starts with a goal; takes steps toward that goal; suffers setbacks; deals with those setbacks. If he doesn’t, he’s not a character, just a prop. You can do all those things with a fully developed, mature character.
I think the way Jackson changed Aragorn’s character did disservice to Tolkien by making Aragorn come across like a whiny loser. But I think what he was trying to do - show the audience what Aragorn was trying to accomplish - was the right idea. I just wish he’d understood Tolkien a little better and trusted the audience more.