At what age should a child switch from whole milk to 2%?

One year: According to the american heart association, children after a year should drink skim or fat-free milk.

No less than 2: The brain of an infant is growing so rapidly in the first 2 years that there is a need for a continuous supply of cholesterol & fat to sustain it. Whole milk provides that support. After 2 the rate has slowed & other dietary sources will be adequate. Whatever the family is normally using will be fine.