Another key risk is inflation, the inevitable increase in the cost of goods and services, including housing, clothing, food, electronic devices and health care. Even with very low rates of inflation, say 3 percent a year, you would lose half of your purchasing power over two decades, Mervine says."That's probably the biggest risk to investors' long-term ability to make ends meet," he says.

To guard against inflation, you can invest in inflation-protected securities or other investments that will gain value as overall prices climb. For instance, stocks in your portfolio aimed at growth rather than income will provide a hedge against inflation, says Michael Reese, Certified Financial Planner and founder of Centennial Wealth Advisory based in Traverse City, Mich.