You can get it after 18 providing your child is doing A levels or something similar but not for university courses and the like...but as said you will have to let them know just what the situation is....here's a quote.....
"If your child is staying in education
You can get Child Tax Credit for your child up to their 20th birthday, if they're in education that counts for Child Tax Credit.
Education counts for Child Tax Credit as long as it's full-time, 'non-advanced' education. Your child needs to have started, enrolled or been accepted onto a course that counts before their 19th birthday.
Full-time, non-advanced education will usually be in a school or college, studying for qualifications like:
A levels
Scottish Highers
NVQ at Level 3
You can't get Child Tax Credit if your child is doing:
higher education, such as a course at university level
education provided by their employer
education provided through any office they hold - for example if your child has an official role such as scout leader or councillor, and the education is provided as part of that role
'Full-time' means they're taught or supervised for more than an average of 12 hours a week during term time."
Hope this helps,