Excuse the aide-memoire, as I never ever recall what the different countries' grades and year systems are:

Key Stage 2, ages 7 - 11, UK years 3 - 6, Scots ?, US ?
Key Stage 3, ages 11 - 14, UK years 7 - 9, Scots P7 - S2, US middle school grades 6 - 8;
Key Stage 4, ages 14 - 16, UK years 10 - 11, Scots S3 - S4, US high school grades 9 - 10;
Key Stage 5, ages 16 - 18, UK years 12 - 13, Scots S5 - S6, US high school grades 11 - 12.

The intention is to easier fillet other people's minds by knowing what to google for...