american school system ages

At the age of five or six, the children attend elementary school (also known as grade school or grammar school), which last six years. The fist year at elementary school is called kindergarten.
In the USA, children start school when they are five or six years old. Depending on the state, schooling is compulsory until the age of 16 or 18. Children younger than five can go to a nursery school or preschool.

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References:

http://www.goodschoolsguide.co.uk/international/transitions/from-american-schools-to-british
http://www.goodschoolsguide.co.uk/choosing-a-school/independent-schools/uk-independent-schools-explained

key stages explained

Key Stage (KS5)
Age 11–14. Y7–9. 1st–3rd form secondary

Key Stage 1 (primary school Years 3 and 4) builds on Foundation Stage experiences, and Key Stage 2 (primary school Years 5, 6 and 7) builds on Key Stage 1 experiences. As children progress through these stages, they move from informal to more formal learning.
The Northern Ireland Curriculum aims to empower young people to achieve their potential and to make informed and responsible decisions throughout their lives. The learning opportunities provided should help young people to develop as individuals, contributors to society and contributors to the economy and environment.

Derby Virtual School works with children from foundation stage onwards. All transitions can be difficult for children in care, but the move from primary school to secondary school (from key stage 2 to key stage 3) can be especially daunting.
The Virtual School supports this transition and offers advice to schools about additional transition support for children in care.

References:

http://ccea.org.uk/key-stages-1-2
http://www.derby.gov.uk/education-and-learning/derby-virtual-school/information-social-workers-foster-carers/key-states-explained/
http://www.goodschoolsguide.co.uk/choosing-a-school/state-schools