I'm wondering the same thing - most adventure books and movies aimed at kids have youthful, energetic characters, but they're not required, and there's plenty of kid's books that don't even have that. Wind in the Willows, anyone? Mole, Ratty, Badger, and Toad? All middle-aged Victorian gentlemen of one ilk or another. Charlie and the Chocolate Factory? Youthful, yes. Vibrant and energetic? No. Explicitly so, as a matter of fact, because he's been slowly starving to death. Grandpa Joe is far more of a memorable character than Charlie.

There's even two other Miyazaki movies - Porco Rosso and Omoide Poroporo which don't have youthful, vibrant, energetic main characters, and which are better than Howl (which is still good - even when Miyazaki fall flat, he's still damn good).

PR eventually gets a young girl as foil, but not until halfway through the movie and she's never the main character. OP flashes back to her childhood a lot, but you never forget that she's 27 now and leading a boring life as an office lady.