The average maturity of US debt is about 4.6 years. Trust Fund Policy requires that funds be invested with a maturity evenly distributed from 1 to 15 years. In 2012, the average maturity of the fund was just over 6.3 years. These average maturities will vary by a year or so depending on economic conditions but there is no reason to believe that the small difference in maturities will cause any problem.
Furthermore the trust fund is still growing. The value of the assets have increased every year since 1981. In 2012, total receipts were 840,190 billion. Total benefits paid were 785,781 billion. The fund increased by 54,409 billion. It's current value for 2012 stands at 2,732,334 billion.
The potential problem is that congress will not act to increase revenues or decrease benefits paid and the fund will begin to shrink. If Congress were to do absolutely nothing, the fund will begin shrinking in value in a few years and would be exhausted in about 25 or 30 years. At that time, benefits would have to be reduced by 20% to 30% to equal Social Security Tax collections. Because of the number of people that depend on Social Security, that obviously will not happen. Either taxes will go up, benefits will go down or some combination of two.
There is not need to reduce the national debt. We have been refinancing and increasing debt for over 50 years and we will continue to do so. The problem is the rate of increase. It is exceeding GDP growth. That has to change.
Trust Fund Data
http://www.ssa.gov/policy/docs/ssb/v45n1/v45n1p3.pdf