What did the children of Raveloe call Silas? Why?

The children always called him "Old Master Marner." Even though he was not yet 40, he had a look about him that made the Silas resemble an old man,

"The prominent eyes that used to look trusting and dreamy, now looked as if they had been made to see only one kind of thing that was very small, like tiny grain, for which they hunted everywhere: and he was so withered and yellow, that, though he was not yet forty, the children always called him "Old Master Marner".