What you don’t know can’t hurt you, according to the old saying. When it comes to retirement planning, though, the old saying doesn’t apply.   Most people understand, at least superficially, that the sooner they begin planning for retirement the better. Putting money aside now rather than later means that you will have more assets in the long run. What many people don’t know, however, is the amount of money they will need to live on in their golden years. To be specific, they seriously underestimate the possibility that they may outlive their assets, according to experts at Wharton. Indeed, Olivia S. Mitchell , professor of insurance and risk management, has this strong dose of advice: It may be a good idea for people to assume that they will need the same level of income during their retirement years that they need now.…