What makes one a Mennonite, according to Jem?

From the text:

“Yonder’s some Mennonites,” Jem said to Dill. “They don’t have buttons.” They lived deep in the woods, did most of their trading across the river, and rarely came to Maycomb. Dill was interested. “They’ve all got blue eyes,” Jem explained, “and the men can’t shave after they marry. Their wives like for ‘em to tickle ’em with their beards.”