A paper found that young adults don't need to start saving for retirement until age 40. The study is based on "total life satisfaction" rather than savings. But, hang on. Let's take a closer look at this and see what actually happens if you wait until 40 to start saving. Who do you think would be more miserable? The person who starts saving before age 40 or the person who doesn't...? We'll break it down by the numbers and see what we can see.