Why is Lenina so startled by the old man and the women nursing babies in Brave New World?

Aldous Huxley's novel Brave New World is defined by its overarching theme of culture clash or cultural conflict. Lenina, a citizen of the modern society that the majority population with Brave New World ascribes to, experiences a degree of cultural shock when she visits an indigenous reservation with Bernard. 

On the reservation, she and Bernard encounter for the first time two startling types of people: an old man and a nursing woman with her baby. These interactions evoke disgust and pity within Lenina, who has never encountered age nor maternal relations before. This is because these types of human interaction and development have been phased out of the society of the Brave New World. Old age has essentially been programmed out of humans with biological engineering, and children are produced in batches in factories rather than by women. Her shock is the result of culture clash.