Personally I don't recall my Commonwealth pass book pre adulthood, came by way of my local school or if any of the schools I attended engaged in the scheme ... Actually I had to check and my Mother says she took the initiative on her own, so myself and other siblings around my age each had an account - not much in them, but already in place for adulthood years down the track.
The scheme was aimed at primary school kids and IIRC not to secondary schools (generally where most are 13 and older.) I also vaguely recall early 00's a couple the parents recounting some issue regarding either the closing of their kid's account or missing funds from the scheme - their kids hadn't even completed secondary school ... but one thing to take in account (up until the late 70s or early 80s) not all schools in my state participated in this scheme (if not the case then something like the principal or P&C sidelined it) and people do move regions to follow their job or to another one, so could see a few years of inactivity with the child still attending school.
The Commonwealth Bank though was good in the fact most local post offices could process their passbook banking, so it made them well suited in more rural areas.
They earned the polite term scam for continuing a scheme that given the opening financial markets in the 90s, where is should have become fairly obvious pushing it harder on schools during the same period, was more for the benefit of the inevitable inactive accounts would bring the bank once parents moved to more competitive banking services - it's one thing to do this to people who set up the account themselves - but targeting those who had it thrust onto them as a kid ... but I'm rather unkind and biased for other reasons. (People will wonder so #1 reason would be the manner of refusal for a loan (by the bank manager and not loans officer) around the business idea of a printing service for bespoke brochures and newsletters based on Ventura software and a couple of really good printers, considering the only competition at the time were spirit duplicator [1] based businesses.)
[1] https://en.wikipedia.org/wiki/Spirit_duplicator
Edit, with the child not which