The future is here: More than 25 percent of the budget that President Carter sent to Congress yesterday is aid to the elderly.

According to a special analysis by the Office of Management and Budget, $155.1 billion out of the $615.8 billion that Carter recommended spending in the next fiscal year would be for the benefit of people 65 and over.

People in this age group represent about 11 percent of the population. Their share of the proposed budget is a few billion dollars higher than the Pentagon's.

The largest single program for the elderly, and the largest program in the budget, is Social Security. In the year beginning next Oct. 1, it will distribute between $91 billion and $94 billion to persons 65 and over -- retirees and their over-64 dependents and survivors.

Next in size are Medicare and Medicaid. Medicare next year will pay $31 billion to providers of medical care to the elderly. Medicaid -- health care for the poor -- will lay out another $5.5 billion for elderly beneficiaries.

The remaining aid comes from assorted smaller programs.

The elderly's share of the budget was only 17 percent in 1969. It has risen since, in part as Congress has moved to lift the living standards of older people.

In 1969, about 25 percent of the population 65 and over lived in what the government officially defined as poverty.

By 1978, the last year for which figures are available, that figure had been reduced to 14 percent, thanks largely to increases in Social Security benefits. But 14 percent was still three percentage points higher than the poverty rate for the population as a whole.

Now the elderly's share of the budget is likely to rise further for demographic reasons. As the baby boom generation born after World War II reaches old age, the percentage of the population over 65 will rise. wBy the year 2030 it is expected to be about 18.3 percent.

Basically, the aid-to-the-aged programs are a form of income redistribution across generational lines. The government takes from younger people to support the older.

Some experts think that the politics of this may become strained in future years.

Today there are three workers paying Social Security taxes for every retiree collecting benefits, and workers already are restive at the Social Security taxes they are required to pay. But in another 50 years there are likely to be only two workers supporting each retiree; the tax burden will be even greater.

Social planners already are looking for ways to ease that burden. One proposal has been to keep workers working longer instead of retiring in their early 60s or at 65. Some planners have advocated raising the "normal" retirement age for Social Security to 68.

The argument is made that with improvements in health and vigor resulting from medical discoveries and better nutrition, people will be able to work to a later age. There also are proposals to limit future increases in Social Security benefits. Other advocate spurring the growth of private pensions and savings plans so that people will have larger nest-eggs of their own to meet old age and will have to rely less on the government.

In addition to Social Security, Medicare and Medicaid, OMB listed these other programs for the elderly in the budget:

About $689 million in food stamps.

About $2.5 billion from the various federal housing programs for the poor.

About $3.7 billion in pensions to veterans 65 and over.

Some $14.8 billion in retirement and disability benefits for over-64 retired federal employees, members of the armed forces and certain other groups.

Another $1.8 billion will go to elderly persons with low incomes under Supplemental Security Income, the welfare program for the aged, blind and disabled.

A variety of federal health programs, in addition to Medicaid and Medicare, for the elderly will total another $2.3 billion.

There are also smaller items, such as the programs under the Administration on Aging ($726 million), the Action volunteer program for the elderly (about $93 million), various senior job programs ($268 million), and special research ($69 million for the National Institute on Aging).

These are expenditures where the portion going to the aged can readily be identified. Additional funds go to the aged under the fuel-aid program for the poor and the program of Social Services grants to the states, plus many other programs, but no age breaddown is available for these.