How to count unique entries that meet two criteria

Hi I am trying to count all records that fall between the ages of 14 - 18.

The ID numbers relating to each record may appear more than once on a long list so I need to count each ID only once.

Example attached.

EG an ID (relating to a person) may appear 3 times in a list of 200. I want to count them if they are between age 14 and 18 but I only want to count them once, not three times.

Thanks - example attached