Could the US implement something like Australia’s superannuation? Every year, x% of your wage is put into a fund you choose, which invests according to your choice of strategy. You’re not allowed to access it until retirement age, and you get tax benefits for contributing to it. Would it be unconstitutional to force employers to put aside wages like that?
That's opting into a 401k, essentially. Employers often match employee contributions to some amount. But it's not compulsory nor is it universally available. IRAs are, however.