Don Lynn had calculated his retirement income down to the last penny. Beginning next year, the former high school mathematics and economics teacher would collect $478 a month in Social Security benefits, enough to allow his wife to retire early. They would spend their time together gardening, taking an art course or two, or traveling.

Ike a growing number of Americans, Lynn did not want to wait until he was 65 years old to get his first Social Security check. He planned to sign up Jan. 7, 1982 -- the day he turned 62.

But this week, Lynn's calculations -- and the plans of millions of older Americans -- changed when the Reagan administration called for the first significant reduction in the Social Security program since it began 46 years ago.

Reagan's plan would drastically penalize workers who retire at 62 instead of 65, reduce all future benefits by 4 to 10 percent, delay next year's cost-of-living increase and substantially tighten eligibility requirements for disability benefits. The proposal, which is expected to go to Congress within weeks, also would limit Social Security payments to those who receive income from more than one government retirement program, and allow retired Americans to earn extra income without losing Social Security payments.

All of this, the Reagan administration says, must be implemented this year to save the overburdened Social Security system from finanical collapse by the fall of 1982. The changes would balance the budget of the retirement plan that helps support 36 million Americans, and eventually lower Social Security taxes, the administration says.

There would be another fallout of the plan, however, this one for Don Lynn and his wife: Reagan's program would take effect Jan. 1, 1982 -- six days before Lynn would have become eligible for Social Security under the old system.

"I really don't have much choice now," says Lynn, after reviewing a sketch pad of calculations in his suburban Alexandria home. If Reagan's plan is approved, Lynn's $478-a-month benefit check would drop to $330. Says Lynn, "I'll have to wait until 65 now.

From the sprawling suburban homes of Alexandria to Goodwin House, a residence for retired people in Wellington, to the single-room apartments of the elderly in downtown Washington, the impact of Reagan's proposal is just beginning to be understood. But last week, the response of the Washington-area elderly was much the same: they were upset and worried. They felt betrayed by the man many of them had supported for president and who many believed would not tamper with Social Security.

"Everywhere along the line, they seem to cut where people have the least," says Rachel Goetz, a 76-year-old lifelong Democrate who lives in Goodwin House. "How can you compare belt-tightening for the people who can move from a Cadillac to a Toyota to a man who can't afford shoes for his kids?"

The Reagan administration says its proposal would save $17.6 billion in the next four years by encouraging -- its critics say forcing -- elderly Americans to work until age 65. Nearly 70 percent of all workers who filed for benefits last year did so at age 62. Most of them either can continue working or simply wait for their benefits, the administration says.

"The trouble with that thinking," says former Social Security commissioner Robert Ball, "is that you assume that most of these people are retiring voluntarily and that you can, by cutting their benefits, get them to stay on and therefore contribute more to the economy. That's simply not true."

In 1976, for instance, a Social Security report showed 54 percent of the workers seeking Social Security benefits at age 62 were in poor health. Another 20 percent either had lost their jobs or had been forced to retire. Only 17 percent wanted to retire.

Evely A. Blackwell, who has worked for 50 of her 61 years is one of those who wants to retire. The resident of Ninth Street NW began her working career as a waitress, labored as a maid, worked in a cafeteria. She now earns $10,000 a year a counselor at the National Center for Black Aged and has been waiting anxiously to retire.

"I feel like I've been cheated," she says. "All my life, I've been working with the understanding that when I got too old to work, I could stop and just draw the Social Security that I've worked for on all these jobs. They've been hard jobs. I've always thought of is as sort of a personal saving account. I don't like what he's doing."

Besides encouraging people to stay on the job longer, Reagan also has proposed saving nearly $21 billion during the next four year by tightening requirements for workers who claim disability payments. Right now, a person's age, education and work experience are considered when he seeks disability payments, but Reagan wants only a person's medical condition to be considered.

"Disability payments are known to be a major government ripoff," an addministration spokesman explains. "As many as 20 percent of the people now claiming to be disabled really aren't disabled at all."

Gaddie Little, 54, of Northeast Washington, believes the Social Security disability program is already tight enough. He says he worked for 35 years as a carpenter and then developed unpredictabel seizures that forced him to retire on disability at age 54. After getting checks for five months, Little was told the checks would be stopped until his case is reviewed next week, leaving him with only a $180 monthly welfare check to pay his bills, including his $115 rent.

To supplement his check, he collects aluminum cans and hauls them to a recycling plant, earning $40 a month. "And I'm not out there picking those cans up by myself," he says.

Little is part of an already implemented Social Security effort to cut the number of people receiving disability payments without deserving it, and it is unclear whether the Reagan plans would affect him. But the president's proposal would tighten it even further by requiring, among other things, that a person suffer an ailment at least two years in order to qualify for disability.

Reagan's critics claim that thousands of Americans who cannot qualify for welfare because they have assets, will be rejected under the tighter guidelines and be forced to continue working despite their ailments or deplete their savings and go on welfare.

Regan also has called for a lowering of Social Security payments to some who also receive federal pensions.

A former government official who lives in Alexandria, but asks that his name not be used, receives $113.40 a month from Social Security for his three children and gets $218 for himself. He also collects another $1,486 a month because he worked for the CIA for 21 years.

If he were applying for Social Security under the Reagan proposal, he would lose his children's $113.40 benefit check, because he retired at 62. Under the Reagan proposal, anyone who retires at 62 loses children's benefits.

The Reagan plan also would require Social Security to take the man's CIA pension into consideration when deciding how much Social Security he would receive.

Not all of those on Social Security and other government pensions are well off, however. Reagan's proposal would also immediately eliminate minimum benefits paid to 2 million people whose past Social Security contributions were very low because they contributed to the program only a short time. Congress is already phasing out these payments, but Reagan is calling for their immediate end, which would result in a $1 billion saving.

Sixty-eigh-year old Doris is a woman with a government pension and Social Security who would feel the immediate impact of Reagan's proposal. She began working for the telephone company but stayed only a short time. She quit work, married and raised a family. Then she went back to work for the government. Today, she receives $147 a month in Social Security as a result of her time with the phone company. Under Reagan's plan she would lose that payment immediately, leaving her $244 a month from civil service pensions.

"I feel I deserve my payment even though I didn't pay much in," she says. "I still worked, you known."

A Washington Post/ABC poll shows that the most disturbing aspect of Reagan's proposal to the public is his call for a reduction in all future Social Security benefits. A worker earning an average salary, now $13,800 receives an initial benefit equal to about 41.4 percent of his last paycheck. Under Reagan's plan, that would drop to 38 percent in the next five years, cutting into the lifestyles of those already retired on Social Security.

But James and Louise Brown, both in their late 70s, don't need government statistics to tell them that the "Golden Years" can be lean ones. "If they take away any of the little money we get, I don't know how we'll make it," says Louise Brown. "It's getting to the point where we can't afford a decent living. We just have a mere existence."

Mrs. Brown, who worked as a beautician for 35 years, gets a monthly Social Security check of $113. Her husband, who once operated a trash removal and telephone-answering service, collects $236. Their one-bedroom apartment at Judiciary House, a District government appartment for the eldery, cost $81 a month rent. They spend $100 a month for food, $10 for telephone service, $9 for a life insurance policy, $5 for medicine and $3 for the coin-operated laundry.

The couple qualifies for food stamps, but refuses to take them.

"I guess," says Mrs. Brown, who recently became bedridden, "that we will have to get the stamps to help buy food."

Others live on more. Cynthia Wedel, 73, for example, lives on Social Security, earnings from stocks and bonds and her husband's pension. But even she knows others don't live so well.

"My sister depends on Social Security a lot. She's been looking forward to that little cost-of-living raise so much," Wedel says, "For the people who are close to the edge, any kind of a cut or a delay hurts those retirement dreams . . . You just try to go on living. It's a dream that was never real except for a very fortunate few."