It's not. It really only affects those working part time jobs from 15 to the age of 18. Full time work is not allowed in most Australian states until the age of 17.

There's far fewer 15-18 year olds than there are 18-65 year olds. There's also fewer again that can work during school hours.

The idea that teenagers are being paid too much or have an advantage in Australians case and that therefore this should prevent changes to minimum wage laws is just a distraction. It keeps getting brought up by people desperate to push a position but who don't want to argue about the far bigger issue of adults trying to survive on minimum wage. Teenagers are an edge case in this discussion and they really shouldn't be the focus.

I don't know about big, but when I was at school-leaver age I would pretty regularly hear about friends getting way less shifts at their job once they hit 18.

Sort of, it's well known that you're unlikely to get a job at McDonald's as an adult