He is five when he meets Leaf.
She's small and quiet and first hesitates when faced with a playmate, but with a push from her father she shows little hesitation in bold strikes and displays a talent for roughhousing that shakes Green to his core.
"We're rivals now," he tells her after a long day of introductions and running through a near-empty house with the old Growlithe who accompanied Leaf's family to Pallet. "When we're older and we have Pokémon, we're gonna battle, and I'll beat you!"
"Nuh-uh," she says immediately, little hands on her hips. Her lips are set in a stern pout, as though she's offended by his confidence. "I'm gonna beat you! I'm gonna travel and make the best team ever. My dad said so. We'll see who wins then!"
"Fine!" he replies hotly, turning on his heel to march back home.
"Fine," she mocks, and doesn't wait to latch the door behind him.
a/n you asked for leaf/green aoi
you got it