If she was 18 at any time in 2023 she may need to make her own claim as she is no longer your dependant. For clarification please contact the Canada Revenue Agency (CRA) at 1-800-959-8281.

Thank you for choosing TurboTax.

As she is 18 it does not make a difference to her taxes. 
so essentially you can’t claim for your 17yr old as the parent and there is no tax benefit to them making the claim! That doesn’t seem right. 

The CRA will be able to clarify your next steps.

Thank you for choosing TurboTax.

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

Unfortunately, if the child has turned 18 before December 31, 2023, they are recognized to be 18 by the Canada Revenue Agency. The child can now have their own claim on their tax return.

Thank you for choosing TurboTax.