Just like how Prohibition was ended by repealing a constitutional amendment, I’m sure the 2nd amendment can be repealed if there really was a push to get it done. However, I don’t think it will ever happen.
I think the USA cannot have a wholesale ban on guns for self defence due to the relatively high rate of violent crime over there, and number of guns circulating in the country among the general public. I think the gun ownership age should remain at 18 (why should an adult not be able to enjoy the same privileges as someone else? same thing with the drinking age, too).
Australia is a totally different story. Guns have been relatively tightly controlled ever since the colonial days. Handguns have always been tightly controlled and so the only form of arms that people could use for legitimate self defence purposes were shotguns and rifles. As a result, having a gun for self defence never really entered ubiquity apart from on rural properties. The gun laws here work only because guns did not proliferate anywhere near as much among the general public as they do in the USA, and the vast majority of criminals cannot easily source a concealable firearm.
What I think should happen over in the USA is a total restriction on the sale of semi-automatic firearms which hold more than 6 rounds in the magazine. People should be allowed to own firearms, but only if they are appropriately licensed and the guns registered - like you would with a car.
I think there is some merit to permitting people in the USA to carry concealed firearms for self defence in more places than are currently permitted. I think in the case of schools, teachers should be allowed (not forced) to carry their own handgun on their person just in case a school shooting does break out.
The perception I have is that a lot of gun nuts in the United States are of the belief that they need to own a huge arsenal of weapons in case they deem that it is appropriate to instigate an uprising against a tyrannical government, which is just absolute insanity in this day and age.