Family unit is 2 parents and 2 children under 18, When oldest child turns 18, can the parents sill claim the travel expenses for the family of 4...since the parents are paying for everything.
CRA rules are "You can claim a maximum of 2 trips per year for other travel for each member of your household". Nowhere do they state any age restrictions. Here's a little more information on the Northern Residents Deduction for travel.
To claim your Northern Residents Deduction (NRD) for Travel Benefits, you'll need to meet the CRA requirements for claiming the NRD first. If you determine you do qualify, you can claim the Travel Expenses for a trip you took anywhere.
Just remember that you are only allowed to claim the "lowest" amount of the three requirements:
You'll find all the information from CRA on how to calculate this deduction in this link: https://bit.ly/2Oicba6