WHENEVER I read an article about someone who continues to work long after a normal retirement age, I add it to a red folder labeled "Older Achievers." During the past year this once-flat folder has expanded considerably to accommodate charming stories about the famous and the obscure, all united - in my file drawer, at least - by the common bond of late-life employment or creativity.Among the more widely known octogenarians in this very random collection of clippings are 81-year-old Japanese filmmaker Akira Kurosawa, 82-year-old actress Jessica Tandy, who most recently starred in "Driving Miss Daisy," and 86-year-old jazz trumpet player Doc Cheatham, whose musical career spans 70 years. And then there are the unknowns, including Sam Cascio, a 94-year-old bellhop at the Chicago Hilton & Towers. Mr. Cascio, who has worked at the hotel since 1927, still carts guests' luggage four days a week, 6 1/2 hours a day. He refuses to retire because he "might get old." The folder also includes stories about the Rev. James Murphy of Ken Gar, Md., who at 82 has been pastor of the First Baptist Church for 40 years. And Mary Houck, the 88-year-old owner of Mary's Beauty Salon in Montgomery County, Md., who continues to serve customers ranging in age from 67 to 92. Mrs. Houck explains that 15 years ago her doctor told her the secret to a long life was to "never give up working." But while the red folder profiling these energetic members of the "no retirement" set grows thicker, so does a blue folder labeled "Early Retirement." In recent weeks alone, the District of Columbia school system offered 1,300 administrators and teachers an "easy out" early-retirement plan that is paying a $10,000 bonus to everyone who retires by July 1. On the West Coast, 636 professors at the University of California have agreed to accept early retirement. Add to these the countless news stories about corporate golden parachutes and sweetened pensions for people as young as 50, and the pattern is obvious: The definition of "retirement" is more fluid than ever before. For some young retirees, the departure marks the end of a 9-to-5 routine, at least temporarily. The New York-based Commonwealth Fund estimates that 6 million unemployed Americans over the age of 55 want to work. Other "early outs" will simply make a transition to another job or a new career - a pattern that will become increasingly common in the future. The US Department of Labor predicts that people entering the work force now will have four careers in their working lifetime. Not four jobs, but four c areers. Many employers face difficult choices as they seek to make room for a new generation. One department chairman at the University of California, Berkeley, acknowledges the value of bringing in younger faculty members with new ideas. But he also laments the loss of those whose "institutional memory and experience" cannot be readily replaced. When is someone "too old" to work? A new study of two American companies and a British retail chain, commissioned by the Commonwealth Fund, demolishes negative stereotypes about older workers. Those over 50 learned to use computers and other new technology as quickly as young workers, and often proved to be better salespeople. They also missed fewer days of work and stayed with a company longer than their youthful co-workers. As in sports, the best teamwork in a lot of departments of life seems to result from a combination of veterans and rookies. A spread of chronological age becomes an enriching form of multicultural diversity. Further diversity is occurring, thanks to broader social definitions of terms like achievement and activity. The old rigid before-and-after distinction between work and retirement is breaking down. What used to be a hobby becomes a profitable case of self-employment. Amid the lip service to "a thousand points of light," volunteerism is assuming a new dignity - and from crack babies to the increasing tribe of nonagenarians, nobody can deny the need is there. The world can afford no dropouts, and in this sense of continuous contribution and continuous self-expression, older citizens are finding a fuller meaning of what a good life really is, not only for themselves but for the rest of us.