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:
16 years old until the 31 August following their 16th birthday, or
16 or 17 years old who is registered for work, education or training in certain circumstances
Under 20 years and in full-time further education or approved training, so long as this commenced before the age of 19. (Even if such a course finishes, the young person can still be classed as a 'qualifying young person' if enrolled or accepted onto another such course).
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.