Diagram of key stage versus age 

This graph matches key stages to age groups.

Key stage 1 applies to primary school children between the ages of four and six.

Key stage 2 applies to primary school children between the ages of six and 11.

Key stage 3 applies to secondary school pupils between the ages of 11 and 14.

Key stage 4 applies to secondary school pupils between the ages of 14 and 16.