Sentence:  Imagine a country where children do nothing but play until they start compulsory schooling at <WSD>age</WSD> seven. Then, without exception, they attend comprehensives until the <WSD>age</WSD> of 16. Charging school fees is illegal, and so is sorting pupils into ability groups by streaming or setting. 
Meaning of the word marked with <WSD> tags:  The number of full years, months, days, hours, etc., that someone, or something, has been alive.