A herder usually is a shepherd. The term is also sometimes used for a cowherd, though that is more often called a koeherder.
In areas with large moors, you may find this as an occupation for adults. Some farmers who had some some land that was not arable kept a few sheep and hired a young boy, a herdersjongen [shepherd boy] to herd them.

Shepherd watching his flock, circa 1654. Credits: Gesina ter Borch, collection Rijksmuseum (public domain)