Fixed mortgage rates fell this week to the lowest levels in six decades. But few Americans can take advantage of the rates to refinance or buy a home.

The average rate for the 30-year fixed mortgage fell to 4.12 percent, down from 4.22 percent, Freddie Mac said Thursday. It's the lowest level on records dating back to 1971. And Freddie Mac says the last time rates were cheaper was 1951, when most long-term home loans lasted just 20 or 25 years.

The average rate on a 15-year fixed mortgage, a popular refinancing option, fell to 3.33 percent from 3.39 percent. That's the lowest on records dating to 1991 and likely the lowest ever, according to economists.

Mortgage rates tend to track the yield on the 10-year Treasury note, which fell to an all-time low this week. An uncertain outlook for the U.S. economy has led many investors to shift money out of stocks and into the safety of Treasurys, lowering the yield.

Still, few expect record-low rates to energize the depressed home market. Over the past year, the average rate on the 30-year fixed mortgage has been below 5 percent for all but two weeks. That compares with five years ago, when the average 30-year fixed rate was near 6.5 percent. Yet prices and sales remain unhealthy and are holding back the overall economy.

Sales of new homes are on pace to finish the year as the lowest on records dating back a half-century. The pace of re-sales is shaping up to be the worst in 14 years.

Many Americans can't take advantage of this rare opportunity. Some are in no position to buy. Unemployment is high, few Americans are getting raises and many are struggling to shrink their debt loads.

But others can't qualify for the lowest rates. Banks are insisting on credit scores above 700 and 20 percent down payments for first-time buyers. Many repeat buyers have too little equity invested in their homes to meet loan requirements.

Roughly 40 percent of U.S. households have the necessary credit scores to get a prime mortgage rate, according to an Associated Press analysis of Fair Isaac Corp., or FICO, data. But that's not the chief reason people aren't buying homes or refinancing.

Just half of Americans say they'll ever be able to save enough money for any type of down payment, let alone one as high as 20 percent, according to a survey by the National Foundation for Credit Counseling.

Nearly a third of homeowners have nearly zero equity or are underwater in their mortgage, according to the real estate research firm CoreLogic. That leaves then unable to refinance because of lender-imposed limits and the cost of extra fees.