Parents are currently advised to switch toddlers to reduced-fat milk at age 2. David M. Goehring/Via Flickr hide caption

itoggle caption David M. Goehring/Via Flickr

Thousands of years ago, a mutation in the human genome allowed many adults to digest lactose and drink milk. iStockphoto.com hide caption

itoggle caption iStockphoto.com

Dairy farmer Bob Andrews feeds heifers in the same barn his grandfather used. He says today "the harder you work, the further you get behind." David Sommerstein/NCPR hide caption

itoggle caption David Sommerstein/NCPR

European dairy farmers spray police officers with milk during a demonstration outside the European Parliament in Brussels on Monday. Geert Vanden Wijngaert/AP hide caption

itoggle caption Geert Vanden Wijngaert/AP

Laura Chase, a member of the MOOMilk dairy cooperative, sweeps her barn in a film still from documentary, Betting The Farm. Pull-Start Pictures hide caption

itoggle caption Pull-Start Pictures

A customer studies a display of milk and dairy products at Bryan's Grocery in San Francisco in March. Justin Sullivan/Getty Images hide caption

itoggle caption Justin Sullivan/Getty Images

In one piece from the edgy campaign, a henpecked husband cowers with cartons of milk in his arms. California Milk Processor Board hide caption

itoggle caption California Milk Processor Board