Child Benefit, Child Tax Credit and Universal Credit can include a 16-19 year old as your dependent if they count as a ‘Qualifying Young Person’.
A 'Qualifying Young Person' is someone who is:
Young people in higher education do not count as qualifying young persons. The maximum upper age limit is 20.
The rules for entitlement to Child Tax Credit and Universal Credit are the same as for Child Benefit. Anyone who counts as a 'qualifying young person' is eligible for inclusion in a claim. So if you (or your partner) are not claiming Child Benefit for a child then you should normally not include them in your benefit claims and should not include them as a child in the calculations you do on the Benefits Calculator.