Fullmetal Alchemist is the gold standard for what a 'teen' story should be. It doesn't talk down to its audience, and it doesn't shy away from the fact that life can be brutal and unfair. The 2001 manga is just as readable today as it was twenty years ago because the art is clean and the pacing is relentless.
It’s a story about consequences. The protagonists start the series by making a massive, arrogant mistake, and the rest of the volumes are about them trying to fix it without making things worse. It’s the opposite of 'brain rot'—it requires you to actually remember names, political factions, and the laws of its universe. If your kid is ready for something deeper than a typical superhero comic, this is the one. Just be ready to talk about some of the heavier deaths; they stick with you.