1. When do you need the money?
If you're not planning to retire for decades, you have plenty of time to ride out any temporary downturns in your account balance. So you can afford to take on more risk in the hopes of getting higher returns.
If you're getting close to retirement, however, you won't have as much time to wait for the market to bounce back if it hits a rough patch. In this case, you might be better off in an asset mix with lower risk.
But be careful not to be too conservative—your account needs to continue growing enough to last you through several decades of retirement!
2. How much risk are you comfortable with?
Some people can easily ignore the day-to-day changes in account balance that sometimes come with more aggressive investments. Instead, they focus on the overall progress toward their goal. Others lie awake at night fretting about what their investments will do tomorrow.
It won't do you any good to constantly worry about your investment decisions, and it probably won't be much fun either. So choose a level of risk you know you can live with.