The Gray Ceiling is purely a function of mathematics. Jon Ciampi, for example, was born in 1973, when the birthrate hit a quarter-century low. Just ahead of him and his peers is the anomaly known as the baby boom, the 77 million Americans born between 1946 and 1964.

Just behind him are the boomers' children, known as Gen Y, who form a second bulge. And sandwiched in between is the baby bust, or Generation X. Known variously as the laziest generation and the most entrepreneurial, they are unambiguously the smallest generation since the Great Depression.

Though that worked to the benefit of Gen Xers when it came to slots in elite schools - and will once again work to their benefit when the boomers finally leave the workforce - right now it's holding them back.

For starters, the workplace makeup has changed dramatically from just a decade ago. In 1996 there were 64 million U.S. workers between the ages of 30 and 39 and only 43 million ages 40 to 59. Now the situation has reversed. As of June 2006 there were only 40 million ages 30 to 39 and 69 million workers 40 to 59, according to the Bureau of Labor Statistics.

Nobody is suggesting that all boomers have it easy.

For one thing, as Fortune reported last year in "50 and Fired," those tossed out the door in the latest recession are having a tough time getting back in. That problem and the Gray Ceiling - a term that has been associated with age discrimination in the past but is taking on a new meaning - share a common cause: In today's leaner companies, executive jobs are fewer, and boomers who have hung on to them are in no hurry to let go.

When Korn/Ferry surveyed 2,000 senior-level managers at global companies recently, they found that 44% said they plan to keep working past 64.