C'mon," Apollo said. "How old are you?"
Thalia hesitated, "I don't know."
It was sad, but true. She'd been turned into a tree when she was twelve, but that had been seven years ago. So she should be nineteen, if you went by years. But she still felt like she was twelve, and if you looked at her, she seemed somewhere in between. The best Chiron could figure, she had kept aging while in tree form, but much more slowly.
Apollo tapped his finger to his lips, "You're fifteen, almost sixteen."
"How do you know that?"
"Hey, I'm the god of prophecy. I know stuff."
The Titan's Curse (Percy Jackson and the Olympians, Book 3) by Rick Riordan