I think the distinction is that an applicant over 54 is an adult whereas an applicant under 18 is a minor. The letter I received was pretty clear that NO minors need attend and my interviewer didn't even ask about him, even though I had her show me where his application was joined with mine.
No need to think about the distinction for applicants over 54. They
MUST be interviewed even though they are test-exempt.
In particular, the applicable PDI states:
"All adult grant applicants undergo a PI interview with a citizenship official, usually as part of their written test session. Adult applicants who are not required to meet the language or knowledge requirement are also interviewed."
https://www.canada.ca/en/immigration-refugees-citizenship/corporate/publications-manuals/operational-bulletins-manuals/canadian-citizenship/admininistration/decisions/interviewing-adult-applicants.html
As for "minors:
As for "minors," as I said,
I do not know. I do not follow processing for minor applicants.
Except I now know a little more, given just a little homework. Helps to do the homework.
HOWEVER, as I also noted, I do know there is a distinction between those who are under 14, versus those between the age of 14 and 18 (that is applicants 14 to 17 years old). The latter must personally sign their application (in addition to the parent's signature). Whether this affects who must appear for the test-interview event I did not know. I now know a little more, given just a little homework. Helps to do the homework.
I also know, HOWEVER, that personal anecdotal experiences OFTEN fail to represent how things go for others. The personal anecdotal experience ONLY illustrates how things MIGHT go (since it is an example of what has happened, it indicates the same COULD happen again for another), and while it might coincide and thus illustrate what is likely to happen (the "rule" if you will), the personal experience often does
NOT illustrate what is likely to happen, and
rarely illustrates what for sure will happen for a different applicant. What controls is, of course, the applicable policy or rule . . . which I did not know.
I also know, HOWEVER, the importance of doing a little HOMEWORK before making definitive assertions, especially about how it will for-sure go.
For example: In the meantime, even though (as I noted) I do not ordinarily follow processing for minor applicants, only a LITTLE HOMEWORK reveals that minor applicants aged 14 to 17 "
must be scheduled for a PI interview."
That said, the applicable PDI
RECOMMENDS that the PI interview "
be conducted prior to the ceremony check-in." They may, however, be conducted on a different date (overall indication is that they MAY be conducted concurrent with parent's test-interview event).
Thus, it appears your reported anecdotal experience illustrates the USUAL process, and thus what is LIKELY, but
NOT WHAT WILL FOR-SURE HAPPEN, if the minor is 14 to 17 years of age.
TAKE-AWAYS:
A minor 14 to 17 years old must be interviewed.
This interview, however, will most likely take place just before the oath ceremony check-in.
The minor 14 to 17 years old may be scheduled for the interview at some other time; it appears likely this would usually coincide with the parent's scheduled test-interview event.
Otherwise, ALL ADULT applicants must be interviewed and this will ordinarily be scheduled concurrently with a test-interview event, including for adult applicants over 54 who are test and language exempt.
Some links to relevant PDIs:
For PDI about "Citizenship: Interviewing adult applicants" see
https://www.canada.ca/en/immigration-refugees-citizenship/corporate/publications-manuals/operational-bulletins-manuals/canadian-citizenship/admininistration/decisions/interviewing-adult-applicants.html
For PDI about "Citizenship grants: Minor applying under subsection 5(1)" see
https://www.canada.ca/en/immigration-refugees-citizenship/corporate/publications-manuals/operational-bulletins-manuals/canadian-citizenship/grant/minor-applying-adult.html
For PDI about "Conducting interviews and hearings with minors" see
https://www.canada.ca/en/immigration-refugees-citizenship/corporate/publications-manuals/operational-bulletins-manuals/canadian-citizenship/grant/conducting-interviews-hearings-minors-applications-received-case-processing-centre-sydney-on-after-june-11-2015.html
(Note: both the PDIs about minors state that those 14 to 17 must be interviewed and
recommend this be done prior to oath ceremony check-in.)
Edit to clarify that possible waivers are not addressed above.