More than any one stock or mutual fund pick, the age you start investing will determine how much wealth you build. To illustrate: Mary starts putting away $100 a month when she’s 22. Her money grows at 8% a year, and then after ten years she stops contributing – and let’s her stake grow. John waits until he’s 32 to set aside $100 a month, also growing at 8% per year, and keeps it up until he’s 64 years old. When they both retire at 64, Mary will have $234,600, John will have only $177,400.