QUESTION: My father-in-law recently immigrated from India. Vigorous and ambitious at age 58, he quickly found a part-time job. He enjoys his work and has no plans for retirement, but he wonders how long he must work to qualify for Social Security and if he will qualify for Medicare. His current job has no health-care benefits.

ANSWER: Your father-in-law must work a minimum of 10 years to qualify for Social Security retirement benefits. Here's why:To be fully insured, your father-in-law must earn 40 Social Security "credits." This year, he'll receive one credit for every $520 he earns, up to a maximum of four credits if he earns $2,080 or more. (Next year, he'll have to earn slightly more as the cost of a credit increases automatically each year as average wages increase.) Due to the four-credit annual maximum, it takes at least 10 years to receive 40 credits.

Excess earnings do not carry over from year to year. For example, if your father-in-law earns $2,000 this year, he'll receive three Social Security credits based on the first $1,560 of earnings (3 X $520

$1,560). But he will receive no credit for the remaining $440, nor can he use that amount to earn credits in the following year.

People eligible to receive Social Security benefits also qualify for Medicare, provided they are 65 or older. People who haven't earned enough work credits to qualify for Social Security may buy Medicare insurance after they reach 65. Legal immigrants 65 and over must live in this country for five continuous years before they can buy into the program.

QUESTION: My parents both have osteoarthritis and spend hundreds of dollars annually on what they hope will be the latest cure. I'm sympathetic but feel they must learn to manage their disease, rather than chase illusive cures. What do you think?

ANSWER: Many osteoarthritis sufferers spend lavishly for supposed cures. The Arthritis Foundation estimates that, in 1980 alone, $95 million was spent on unsound remedies.

There is no cure for osteoarthritis, but health promotion activities can help. Patients who took part in a four-year study at Stanford University on arthritis self-management reported that pain, dependency and depression were reduced.

The Arthritis Foundation recommends that osteoarthritis sufferers beware of products that:

-claim to work for all types of arthritis;

-use only case histories or testimonials as proof of effectiveness;

-do not list ingredients;

-do not mention potential side effects;

-are based on a "secret formula";

-are available through only one supplier;

-are promoted only on television or by mail order; or

-claim to cure arthritis.

If you feel your parents have been ripped off, write to the Food and Drug Administration, 5600 Fisher Lane, Rockville, MD 20857. Complaints concerning products advertised on radio or television should be filed with the Federal Trade Commission, Sixth Street and Pennsylvania Avenue, Washington, DC 20580. Urge your parents to demand that the government take a more active role in protecting consumers.

According to the National Resource Center on Health Promotion, arthritis is the nation's leading chronic disability. It affects 17 million Americans.

The Arthritis Foundation of America, with chapters in every state, provides services and education for people with arthritis and publishes many useful pamphlets. For more information, write to the Arthritis Foundation, P.O. Box 19000, Atlanta, GA 30326 or call toll-free, 1 (800) 283-7800.

QUESTION: I've always loved music but never had the chance to develop my talent. At my age , would voice lessons be a waste of time and money?

ANSWER: People of any age can experience joy and satisfaction when they tap their creative potential. Bill Elliott ought to know. As a performer, his career included a stint as a member of the acclaimed Fred Waring Pennsylvanians. As a teacher, he has taught singing to older adults through a special program of Santa Monica College in Southern California, on a non-credit, free-of-charge basis.

Most of Elliott's students are beginners who develop confidence in their vocal abilities through group singing and vocal exercises. Elliott says his students share a common attitude: "They're responding to a little voice inside that says `I want to keep trying.' "

Though most of his students sing for their own enjoyment, one woman in her late 60s, whose voice Elliott calls "a natural," recently won second prize in a Los Angeles senior talent competition. Another student, age 84, came to a class on a whim and is now so enthused about her progress that she is taking private lessons.

For opportunities to explore your musical talents, check with your local senior center, community college, high school adult-education division, theater group or house of worship.

Send questions about growing older to On Aging, P.O. Box 84256, Los Angeles, CA 90073. Questions of general interest will be answered in the column; individual answers cannot be provided.