Part of what separates the funds is their approach to handling the period after the retirement date. Some funds reach their final allocation on the retirement date. After that the allocation stays fixed -- no matter how many years an investor holds the account. Other funds continue lowering their stock allocation after the retirement date. Among the more aggressive choices for retirees is American Century LIVESTRONG 2055 ( AREMX). The fund starts with about 85% of assets in stocks. When the portfolio approaches the retirement day, the stock allocation reaches 45% and then stays put. Wells Fargo Advantage DJ Target 2050 ( WFQFX) takes a more cautious approach. The fund starts with about 90% of assets in stocks. But by the retirement date, there will only be 23% of assets in stocks. The allocation keeps shifting so that five years after retirement only 15% of assets are in stocks. Faced with volatile markets in recent years, some funds have been tinkering with their allocations, shifting to more conservative approaches. But one manager that has remained constant throughout is T. Rowe Price. The company's 2055 fund has about 90% of assets in stocks. At retirement the allocation is scheduled to shift to 55%. The figure keeps falling, reaching 40% 10 years after retirement. T. Rowe Price argues that its clients should prepare for retirements that can last 30 years. To provide income for such an extended period, retirees should own stocks that promise to deliver long-term capital gains. Either a conservative or aggressive approach can work. But savers must shop carefully to avoid picking a fund that may produce a nasty surprise the next time that the markets drop.