Hi Kivacin, for your question, i think its explained on these page from CRA

https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-re... (For AGE eligibility)
 

https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-re...  (SEE EXAMPLE 5) 
 

Example 5 at the bottom of the page explains how it works.  In the example Edward (name 1) is claiming the standard amount of $1,200 (amount 1) for each of the four travelers. For line C, the total amount in row B is identified as $1,200 for each traveler.

If there was more than 1 person claiming a part of the amount of 1200$, then the total  can not be more than 1200.
 

So in Chart B, column 3  , it if there was more than 1 trip, the amount of 1200$ would be lower, depending how they decide to split

As per the age, for a child its 18

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
another individual who is wholly dependent for support on you, your spouse or common-law partner, or both and who is either:

your or your spouse's or common-law partner's parent or grandparent
related to you and wholly dependent by reason of mental or physical infirmity

Complete Chart A in Step 3 of Form T2222, Northern Residents Deductions to identify yourself and any eligible family members who lived with you during the period(s) indicated in Step 1. Show how you allocated each individual's $1,200 standard amount (if applicable).

Complete Chart B to calculate your travel deduction.