It's not. Any investment program needs to consider the rate of return, your risk tolerance, and your time horizon. Young people investing for retirement have a long-term time horizon. Investing in stock mutual funds makes sense. When you are closer to retirement, more conservative investments such as bonds and cash may be more appropriate.
Not much different from saving for college—or saving for a house. The process is the same. What is your goal? What do you have to work with? When will you need the money? Getting clear about your goals always eliminates unwanted or unnecessary alternatives.