Sounds like Questors (2007) by Joan Lennon.
Synopsis from Goodreads:
Three worlds, held in perfect balance. Nothing can change that. Well, nothing except a cataclysmic disruption in the Space-Time Continuum...
Luckily the people in charge have a plan: Create three perfect Heroes, the best of each world, and send them on a quest to find the Objects of Power that will restore the balance. But things go wrong when the Heroes are needed ten years earlier than expected, and three confused kids set off to save the worlds. Madlen, Bryn, and Cam have no idea what they're looking for or where they'll find it. What they do know is that to fail would mean unthinkable disaster.
It's a pity, then, that someone is determined to stop them...
From the icebound city of the dragons to the magical kitchen of The London House, Joan Lennon has crafter a highly inventive story that is fast-paced, fantastical, and funny.
And here's a relevant excerpt from the book itself:
"Look, kid," he said. "I'm not sure how to say this tactfully, but I've known you for, what, hours, right, and I still can't tell..." He ground to a halt. His face was getting red.
"Can't tell what?" said Cam, concentrating on the food.
"I just can't tell...," he started again, and then finished in a rush. "Look, are you my brother or my sister?"
Cam stopped eating and stared. "I'm neither."
Madlen made a choking noise. Bryn turned and saw her watching them. His face got even redder.
"No, you don't get the question," he said, turning back to Cam. "I know this must sound really, really stupid, but -- I don't know what sex you are. You know, boy or girl, pink or blue, that sort of thing. To be honest, I can't really tell." He spread his hands. "Sorry!"
"No," said Cam. "I got the question, and the answer is -- I'm neither. I'm an emergent." And as the other two stared, Cam said, "Give me a chance, will you? I'm only eleven!"
"What are you talking about?" chorused Madlen and Bryn in unison.
Cam looked from one uncomprehending face to the other. "You're Trentorian, right? And you're from Kir. Are you trying to tell me neither of your worlds has emergents?"
"Yes," said Bryn, speaking very slowly. "That could very well be exactly what we're trying to tell you. If we had a clue what you were on about."
The Daldorian leaned back and whistled. "Well, I never. You just assume, don't you, that everything's the same all over. My word..."
"Right! Right... So, emergence -- Well, it's like this. On my World we don't settle on a sex until we hit puberty." Cam paused at a sudden thought. "You do have puberty, don't you?"