<prompt> Human: Worst case scenario is the market crashes when Im 64, and Id have to work a few extra years for it to recover and break even. But if I switch over to bonds at 60, then a potential crash wouldnt be so damaging. The way I see it, the long-term risk of investing in stock funds i.e. SP 500 is low until the very end, so I might as well enjoy the higher net returns. Am I wrong?  <response>: If you were a robot, and had an infinite investment horizon, then your premise would be correct.However, you are human.1. Humans have emotion. People often sell when they are down big in order to prevent further losses. That just locks in the loss. An individuals bond holdings should properly represent their risk aversion. So, in bad years where stocks go down 50, an individuals total portfolio may only decrease by 25. A 25 drop might be easier to stomach without selling than a 50 drop would be.2. Humans have limited lifespans. You do not know how the market is going to play out over the next 30 years. The global equity market could resemble Japan 1989-present. A bond allocation might be helpful in that sort of environment. The trouble is, you just dont know. You dont get to choose what sort of long term market conditions you will live in. This is why diversification is great, you likely wont have the best return possible during your investment timeframe, but if you save enough money, it will very likely be good enough. 