Why were nines given bicycles in The Giver?
The new possessions or arrangements given during the yearly ceremonies recognized the growth and increasing level of responsibility of the community's children as they grew up.
By the age of nine, the children of the community were considered old enough to begin traveling about the community independently. They were expected to begin volunteering their time and services at various locations within the community. They were thought to be old enough to take care of the bicycle and to understand how to ride it safely. Bikes were also an ecologically sound mode of travel and exercise.
Of course, the reality was that most children knew how to ride a bicycle long before they received one during their Ceremony of Nine.
The children all received their bicycles at Nine; they were not allowed to ride bicycles before then. But almost always, the older brothers and sisters had secretly taught the younger ones one. Jonas had been thinking already about teaching Lily.