If travelling domestically alone your 15 year old should be declared to QF (..."You should list your child as a Young Passenger so they are identifiable to crew in the event of a delay or disruption..."): did you do this?!

The irony in this case is that QF would allow the 15 year old to supervise younger children during domestic travel (further rules apply to international travel for obvious reasons).

As others have stated, liquor laws are usually strict and good on QF for applying them.

There are some anomalies in liquor laws (remarkably in the state of QLD you do not have to be over 18 or even 16 to serve alcohol in a bar, for example, only of legal age to work (13 years) and be paid the adult rate of pay according to the minimum standards set by the Hospitality Industry General Award, so your son could have a job in a bar, but not drink or be served in said bar in QLD).

So in this case, QF is (probably) simply applying the legislation in a responsible manner.

It MAY also be the situation that the outbound flight was during hours of non service of alcohol and the inbound when the bar was operational, which is why the lounge agent was more relaxed (although did not follow the rules and on that basis IMHO QF should not have allowed your son into the lounge in SYD.).

There is also the point that QF probably doesn't want folk handing out their loyalty cards to others to claim benefits, which aren't due to that person and / or would be applying due diligence to ensure the card had not be "found" or stolen.

I'm not for a minute suggesting you yourself are a bad parent, but do consider that situations and perceptions differ and what might be the norm and "heathy" in one case does not necessarily apply in another.

For example, deciding to use 15 year old as a "babysitter" in such a public and high activity environment, may reflect parenting styles and individual personalities / maturity levels of kids differ: no problem in many cases (aside the liquor laws).

QF itself allows supervision by a 15 year old when travelling.

On the other, by way of contrast, placing a child into an adult role can have dire unintended psychological consequences in certain specific scenarios (e.g. becoming a surrogate adult partner in separation cases). A former partner was brought up in a household where the kids were expected to be surrogate parents in various respects and this did them no end of lasting psychological harm.

Facilitating maturity and independence and self confidence good - transfer of roles bad. Somewhere balance!

IMHO QF lounge staff shouldn't be accepting the role of responsible adult when you go off duty free shopping and should presumably be informing you not to leave the children unaccompanied in the lounge! Apart from compliance with the liquor legislation, consider what the legal liability would be in the event of an accident or event! QF would likely be unwilling to incur any unnecessary business risk. Since your children are supposed to be accompanied in the lounge it might be that you would be legally responsible for any mishap even though not physically present - perhaps some liability defaults to QF(perhaps a legal brain herein can clarify).

QF has to be legally compliant on the one hand (i.e. manage alcohol responsibly) yet apply balanced rules on the other (e.g. allow accompanied travel with a 15 year old sibling).

FWIW, as traveller, I would not expect to come across children of any age under 18 in a lounge unaccompanied by an adult on the basis of liquor laws and my own personal opinions on responsible parenting. I avoid buying drinks in our local QLD bars / restaurants manned by staff blatantly under 18 years (although it's legal). I would probably bring unaccompanied children in a lounge to the attention of the QF lounge staff. I respect opinions will differ, perhaps strongly, on such matters, and my opinion should not be taken as a personal criticism of the OP.