From US.Gov.:

Congress: Benefits
Members of Congress receive retirement and health benefits under the same plans available to other federal employees. They become vested after five years of full participation.

• Members elected since 1984 are covered by the Federal Employees' Retirement System (FERS). Those elected prior to 1984 were covered by the Civil Service Retirement System (CSRS). In 1984 all members were given the option of remaining with CSRS or switching to FERS.

• As it is for all other federal employees, congressional retirement is funded through taxes and the participants' contributions. Members of Congress under FERS contribute 1.3 percent of their salary into the FERS retirement plan and pay 6.2 percent of their salary in Social Security taxes.

Members of Congress are not eligible for a pension until they reach the age of 50, but only if they've completed 20 years of service. Members are eligible at any age after completing 25 years of service or after they reach the age of 62. Please also note that Member's of Congress have to serve at least 5 years to even receive a pension.

The amount of a Congressperson's pension depends on the years of service and the average of the highest 3 years of his or her salary. By law, the starting amount of a Member's retirement annuity may not exceed 80% of his or her final salary.

Data compiled in 2003 showed 413 retired Members of Congress were receiving federal pensions based fully or in part on their congressional service. The average age of those retiring under CSRS was 75.5 and had at least 20 years of federal service. Those who retired under FERS had an average age of 68.3 years and 21.6 years of federal service. Their average retirement payment was $3,909 a month.