Same issue here. I think turbotax needs to be updated to allow the dependent in the 18th year to be added (re: Northern Travel).
The CRA site is fairly clear:
Line 25500 – Calculate your travel deduction
Determine who an eligible family member is
An eligible family member is someone who lived with you at the time of the trip and was one of the following:
- your spouse or common-law partner
- your or your spouse's or common-law partner's child under the age of 18
