If you're saving for your own retirement, you might want to consider the risk of dying before you can spend that money. A 25-year old male has an 18.6% chance of dying before he reaches 65, so he should discount his expected return accordingly.

Note that the discount is in the direction of how much you value the assets in your estate after death. ie. Altruistic (and nepotistic) preferences still apply so do not simply multiply by (1-0.186).