The old rule of thumb used to be that you should subtract your age from 100 - and that's the percentage of your portfolio that you should keep in stocks. For example, if you're 30, you should keep 70% of your portfolio in stocks. If you're 70, you should keep 30% of your portfolio in stocks.
However, with Americans living longer and longer, many financial planners are now recommending that the rule should be closer to 110 or 120 minus your age. That's because if you need to make your money last longer, you'll need the extra growth that stocks can provide.
To find the right asset allocation for you, go to our asset allocation calculator.