why do people think charles wallace is dumb

Meg relates how many people assume that Charles Wallace was dumb, or slow, because he hadn’t started speaking until he was four years old.

"It was true that Charles Wallace seldom spoke when anybody was around, so that many people thought he'd never learned to talk. And it was true that he hadn't talked at all until he was almost four. Meg would turn white with fury when people looked at him and clucked, shaking their heads sadly."

People thought that Charles Wallace was dumb because he wouldn't talk to anyone outside his family.