Boston — Sixty-five. The number is somehow sacrosanct. It has come to symbolize retirement itself. But the legal definitions and societal constraints that have pegged age 65 as the time at which a ''productive'' worker becomes a ''postproductive'' worker are slowly being altered, and for those still on the job, the changes in attitude may have considerable effect on future financial and career planning.
The impetus to rethink attitudes toward retirement are the result of changes in population, economics, and longevity.
Studies show that by the year 2,000, 1 out of 5 Americans will be over the age of 65, or a total of 32 million people, up from 24 million today. This population shift, combined with a shrinking work force and the possibility of continuing high inflation, spells out a squeeze in government benefits for retirees in coming years.
At present, retirement at or before the age of 65 continues to be a popular option. For the last five years 64 percent of all new retirees have been under the age of 65. The reasons most often cited for early retirement are health, declining salary, and the incentives created by government and private pensions to quit the work force.
But the scenario may soon be changing. In 1979 Congress raised the mandatory retirement age from 65 to 70 for most workers. Now pressure is beginning to build on lawmakers to raise from 65 to 68 the age at which a retiree can receive full benefits.
Also, some of the disincentives for continuing to work have been removed. This year retirees can make as much as $5,500 at a part-time position and still receive full benefits.
Raising the social security qualifying age may prove to be one of the more minor changes the system faces. Kenneth McLennan, vice-president and industrial studies director for the Committee for Economic Development believes that to avoid social security deficits of $80 billion to $100 billion by l986, Congress may have to cut indexing of benefits from 100 percent of inflation (as measured by the consumer price index) to as low as 60 percent.
Others are not quite so pessimistic. Merton K. Bernstein, professor of law at Washington University in St. Louis, says he ''rejects the doomsday prognosis for the next century.'' He believes certain assumptions underlying the forecasts for social security are open to question. He adds: ''The long-term outlook is not anywhere near as critical as it has been portrayed.''
The political ramifications of tampering with social security may continue to vex elected officials in Washington, but the root cause of the financial quagmire - inflation - holds a clear message for those still in the work force. That is: Expect to work past the age of 65 whether by choice or economic necessity.
The idea of graduallly phasing in a higher social-secuirity retirement age has gained credible support from lawmakers and private interest groups.
Dr. Herbert Parnes, professor of industrial relations and human resources at Rutgers University, offers a typical view. ''Extending the working age past 65 is not a bad idea provided a person who has to retire for health reasons is allowed to do so.''
Yet prolonging Americans work lives will present complications, chief among them being age discrimination.
Despite an act of Congress in 1967 that outlawed discrimination based on age, the problem today remains ''subtle and insidious'' in the words of Gladys Sprinkle, director of Over 60, a nonprofit placement service for older Americans based in Maryland.
''The law itself hasn't changed anything,'' she says, ''except maybe it has made employers more cautious in their discriminatory practices.'' She cites the case of a man her firm recently handled. At the age of 52 he graduated last year from a major university with a degree in computer science. All of his classmates have found jobs, but after 30 interviews on campus with potential employers he still wasn't offered employment.
Many of those who have fallen victim to age discrimination have pressed their cases in court and won. The results have forced some employers to rethink their hiring policies. The number of cases involving age discrimination have almost tripled from over 3,000 in fiscal 1979 to close to 9,000 in 1981. One quarter of all cases now handled by the Equal Employment Opportunity Commission involve age discrimination.
Yet it is impossible to legislate changes in attitude. Some companies continue to bypass discrimination laws through a variety of methods. Interviews are sometimes conducted over the phone where an applicant's age can be determined and a paper trail avoided. Or, advertising for job openings is done without the use of a company's name, so that when an elderly person responds in writing to a post office box number and is rejected, the job seeker cannot trace the firm involved.
The objections that companies have traditionally had to hiring older workers generally center on the fear that their productivity will decline, and that they will clog up the promotion channels and keep younger workers from moving up through the ranks.
But age discrimination is not universal. For 40 years Bankers Life & Casualty Company has had no mandatory retirement policy. The firm finds that older workers match the productivity of their younger counterparts, and contributed in intangible ways by ''offering integrity, continuity, stabilty, and permanence,'' says a top official
Besides discrimination, there are other factors clouding the future job market for older workers. In 30 years, when the children of the baby boom era are drawing to the close of their working lives, the pool of available jobs for older workers may diminish drastically in the face of rising demand. On the other hand, some experts argue that the overall shrinking of the labor force will actually present older workers with more job opportunities.
Kenneth McLennan of the Committee for Economic Development believes the economy is flexible enough to handle any potential surplus of older workers. ''There are no set number of jobs in the economy,'' he says, adding, ''the way an increasing number of women have been absorbed into the work force in the last decade is a model for the way older workers can be brought into the mainstream.''
So, what can today's workers expect when they reach the age of 65 and what can they do now to avoid a financial crunch later?
Unfortunately, the rising cost of living makes it difficult to even guess. The prospect of continuing high inflation threatens to devalue most retirement plans. ''Inflation has the potential to kill off private pensions as a meaningful contributor to retirement income,'' says says a spokesman for the American Association of Retired Persons.