People under the age of 18 are not counted for council tax purposes. 

If there is only one adult aged 18 or over living in a property - for example a single parent living with his/her children they are entitled to claim single person discount. If a property is occupied only by people under the age of 18, the property is completely exempt from council tax. The exemption stops when someone aged 18 or over moves in, when someone already living there turns 18, or when the property is re-let or sold.