It is not possible for a large nation to save for the future.
The wealth of a nation consists of its economy, millions of people working to produce goods and services. You can’t put that in a box and take it out 50 years later.
An individual can save money, and invest in stocks and bonds while he is young, and draw it out in his old age.
But what is a country going to do? If there are not enough young people producing goods and services, saving consisting of stocks and bonds are not going to be worth anything.
But what is a country going to do?
Answer:
Provide a mechanism wherein all citizens are able/encouraged to do the above. It's called privatization of Social Security and has worked like gangbusters for Chile.
But, you probably already knew that...