True, but he won't be twelve forever. And Penelo's only what, 15? Maybe 16, but even 4 years is not a big age gap. Don't get me wrong, I like Vaan: what he lacks in savoir faire he makes up for with bravery and a determination to prove himself. He has a lot of self-confidence, although it isn't (yet) entirely justified. But Larsa, though only twelve, carries himself with an assurance that is entirely justified. He has power, and he knows how to use it, and he uses it for the good of others. He is an old soul and very wise for his years. It's been a while since I played the game, but I'm pretty sure the one thing everyone agreed on was that Larsa would make a great emperor and be the saving of Ivaiice.
He clearly has a bit of a crush on Penelo, and she doesn't quite know what to make of him. She isn't star-struck by his birth and his prospects, but he does have impeccable manners and he's really useful in a battle. She must think it's kind of cute having this serious little princeling nursing such a crush on her.
Vaan needs to see that other guys appreciate Penelo for the fine woman she will grow up to be.
I think if Penelo and Larsa met again in six years time, the story would be very different. Like Anakin and Padme. Only with better acting. And I would care.