With the relatively fixed retirement age, the dramatic increase in life expectancy and the sharp decline in fertility have caused a serious aging problem and an unsustainable pension crisis. It is therefore necessary to design flexible retirement benefits rules that consider life expectancy. By introducing the lifetime utility optimization model, the closed-form solution for the flexible retirement age is obtained. Pension benefits incentive strategies are constructed to encourage contributors to choose a retirement age that is beneficial toward narrowing the pension gap. The empirical studies show that China will face a serious pension gap in the future if the current statutory retirement age is not adjusted. If the retirement age is delayed according to life expectancy, the future pension gap will be greatly reduced.