I suspect you are filling out a W-4 for a new job and want to know if your child qualifies for the credit? They do not, because your child will not be "under 17" if she is 17 on December 31. Remember that you got the full credit in the year they were born, even though they were only alive half the year. So you don’t get any credit the year they turn 17, even though they were under 17 for half the year.
