Can a person have ADHD even if they're quiet and well-behaved as a kid?

Yes. Many children with ADHD were quiet, compliant, and struggled internally rather than outwardly. This is especially common in inattentive presentations. Instead of disrupting class, these kids daydreamed, zoned out, forgot instructions, or worked much harder than peers to keep up. Because they weren’t causing problems, their struggles were often missed or dismissed. Teachers might describe them as bright but distracted, shy, or inconsistent. Being well behaved does not rule out ADHD. It often just means the symptoms weren’t disruptive enough to trigger evaluation.