I have seen 15 ish years old kids flashing their AMEX cards at the Malls.. so my guess is 15-16 somewhere around that.. but I thought for AMEX cards it has to be 18 since it asks for DOB, etc when adding them on parents account.
I just googled. There is no legal age for AU. It depends on the credit issuer. I guess Amex requires it to be 16. Other companies might have different set of policies. I'll call and find out.