All of this is possible because in this densely populated country distances to destinations are relatively small, and bikes are integrated into the infrastructure of everyday traffic by a dense net of cycle paths. These lanes are not always as nicely asphalted as roads for cars, but things slowly continue to improve. You will find traffic lights and signs especially aimed at cyclists everywhere. Also, everyone in the Netherlands has at least one bicycle. This of course makes it necessary, as a cyclist, to expect and adjust to a variety of cycling-behaviour.
Infants have tiny bikes and have it imprinted in their genes to swerve all over the road.
Luckily mostly parents are present to limit their movements. Old people, on the other hand, tend to drive veeeery slowly, because they are not physically as flexible as they used to be and in general have to deal with less sensory input to use for navigation. In the Netherlands these are allowed to drive alongside each other in rows of two but only as long as they don't hinder other traffic. As when you drive together a conversation is mandatory, they have less attention to devote to navigation, so be careful when you meet such a group. A special class of these group-cyclists are school children.