If you are in the UK the rules are as follows subject to financial eligibility ;
You can get Child Tax Credit if you are 16 or over and you are responsible for at least one child. This means a child under 16, or a young person up to the age of 19 who is in full-time education up to A level or equivalent, or on certain approved training courses. You may also get Child Tax Credit for a young person who has been accepted on one of these courses. You can also get Child Tax Credit for a young person aged under 18 who has registered with the Careers Service, if they have left school within the last 20 weeks.
Hope this helps!