Excuse me in advance for jumping in here.
First of all, you must be at least age 50 to make catch-up contributions.
Second, you must have elected to make the maximum amount of regular contributions for the year ($15,500) to make catch-up contributions.
Third, you do not get matching contributions on catch-up contributions. You get matching contributions on the first 5% of regular contributions ONLY.
From the TSP site:
Questions and Answers as of April 30, 2008
Q1:How much can I contribute to the TSP?
A: The Internal Revenue Code places an annual limit on elective deferrals, e.g., tax-deferred employee contributions to the TSP.
The elective deferral limit is $15,500 for 2008.
Consequently, once you have contributed $15,500 in 2008, you may not make any more (regular) tax-deferred employee contributions for the rest of the year.
For FERS employees, this also means that you will not receive any more Agency Matching Contributions for the rest of the year.
See the Fact Sheet "
Annual Limit on Elective Deferrals" for more information on this limit, including what happens to Agency Matching Contributions when the annual limit has been reached.
Q2:If I am at least age 50 (or if I will become 50 in the calendar year), may I make catch-up contributions?
A: If you have made -- or will make -- the maximum amount of regular employee contributions for the year ($15,500 in 2008), you may also make over-50 catch-up contributions to your TSP account. Catch-up contributions have their own annual limit, which is $5,000 for 2008.
FERS employees do not receive Agency Matching Contributions on catch-up contributions.
For more information about catch-up contributions, see the Fact Sheet "Catch-Up Contributions."