Retirement bites? Almost half of Gen Xers say they'll need a miracle to retire.

The oldest members of Generation X are turning 59 1/2 this month, the earliest age when workers can start withdrawing retirement assets without a penalty. But many Gen Xers are far from prepared for their golden years, with almost half saying it would take a "miracle" for them to be able to retire, according to a new Natixis study.

Gen X — people born between 1965 and 1980 — is the first generation of U.S. workers to come of age with 401(k) plans as their primary retirement vehicle after employers largely shifted away from traditional pensions in the 1980s. But the 401(k) puts the onus squarely on the shoulders of participants to figure out how much to save, how to invest and how to withdraw their money in retirement — a do-it-yourself approach that noted retirement expert Teresa Ghilarducci has described as flimsy. 

That's left Gen Xers largely on their own to plan for retirement, and many are woefully underprepared, not only in the amount of assets they have squirreled away but in their comprehension of key financial information, according to Natixis, an investment bank. The average retirement savings of Gen X households is about $150,000 — far from the roughly $1.5 million that Americans say they need to retire comfortably.