"Youth is the time of getting, middle age of improving, and old age of spending," said poet Anne Bradstreet in the 17th century.

Perhaps that notion was relevant to her time, but this is a time when the older citizen may be locked into a fixed income of dwindling buying power. As a search for senior-citizen discounts revealed, however, there's one compensation for getting older: You can get some things cheaper.

The magic age for qualifying hovers around 60, 62 and 65, but to be on the safe side you need an I.D. or some proof of age. A driver's license, Medicare card, or a Senior Citizen card may suffice in most cases, but for the Metrorail and Metrobus, you must be 65 and carry a card issued by Metro Authorities. Even if you are 100 years of age -- and look it -- the driver will still insist you pay the regular fare if you don't have the Senior Citizen I.D. Card.

To become a bonafide Senior Citizen, get a card from your local office of aging. You must be 60 or over. Always carry your card, even though you won't necessarily be asked to show it. Thus, when you ask for Senior Citizen rates at the movies and go unchallenged by the ticket-taker, don't take offense. You're still saving a few bucks, even though you thought you looked young for your age.

Some people, of course, are reluctant to admit Senior Citizen status. A 64-year-old friend was about to pay her $23.50 beauty salon bill, when the manager asked, "Are you 62?"

"I should say not," she retorted, "I'm 59."

"Too bad," he said. "If you were 62 you could have gotten our 20 per cent discount."

For those who qualify -- and want to admit it -- there are many good Senior Citizen discounts available in the Washington area. One word of caution: Since policies change constantly, always check first. Many places such as movies, theaters, hairdressers, furniture stores, optical work and services like home, TV and radio repair offer Senior-Citizen discounts but do not always advertise that fact. Here is a sampling:

Transportation

Airlines -- Some airlines offer one-third off the regular one-way economy fare when the flight is not heavily booked.

Amtrak -- At 65 you can get a 25 percent discount if one-way fare is $40 or more.

Greyhound and Trailways -- 65 years of age gets you a 10 percent reduction on fares.

Metrobus -- You must be 65 or older and always present a special Metrobus I.D. card, available at all Metro sales outlets and at the Metro Transit Authority Building, 600 5th. St. NW. Or call Metro's Consumer Assistance Office, 637-1328, for the outlet nearest your home. You will also need this card to purchase a Metrorail farecard. In the District you pay 20 cents to ride the bus, and in Maryland and Virginia, 30 cents.

Metrorail -- If you are 65 or older, you can ride Metro for half the rush-hour fare. You'll need a special farecard ($3), available with a Metrobus I.D. card, at the places mentioned above.

Ride-On Bus -- For this Montgomery County-sponsored bus system, Senior Citizens may also show the Metro I.D. card as proof of age. If you are 65 or over, you pay 30 cents instead of 45.

Movies

Most offer dscounts to Seniors 60 and over. Ask before you purchase your ticket, since many theaters do not display signs announcing available discounts. Each movie house has its own arrangement. For example, the K.B. Janus will charge half price before 5:30 p.m. Any of the Circle Theaters will give you a card entitling you to $1 off at any of their theaters charging more than $2 regular admission. The White Flint Theaters in Maryland give half off on movies from Sunday to Thursday. Call to find out if and when discounts are in effect.

Theaters

Some theaters will consider you a Senior Citizen at 60. However, at Arena Stage you need to be 65 to qualify for what they call the Golden Circle. Write to the Arena Stage Box Office, and ask for a Golden Circle Card. With this card as identification you can see plays for $5.50.

Kennedy Center -- With a Medicare card or other proof that you are 65, you can get seats for many concerts and plays at half-price. Discount also available for AFI movies.

Olney Theater -- $1 off all seats except Saturday night. Age 60.

Silver Spring Theater -- $1 off the ticket price. Age 60.

Smithsonian Air & Space Museum -- Half price for "To Fly" and "Living Planet" films. Age 60.

University of Maryland Concert Series -- Discounts offered on either individual or subscription basis. Age 60.

University of Maryland Theater -- Discounts on plays. Age 60.

Restaurants

Hecht Co. Restaurants -- 10% off on meals. Age 60.

Hot Shoppes -- 10% off on specials. Age 60.

McDonald's -- Ask the manager for a Golden Arches card entitling you to a free hot or cold drink with your sandwich or breakfast. Age 60.

Drug Stores

Many neighborhood pharmacies offer discounts. The large chains in the Washington area give customers age 60 and over 10% off on all prescriptions and house-brand products. The American Association of Retired Persons maintains an extensive pharmacy from which you can order (if age 55) either by phone, mail, or in person if you have been a member for at least a year. AARP Pharmacy Service: 1224 24th St. NW, 965-2800.

Miscellaneous

Banks -- More and more banks are offering free checking accounts for seniors. At some, no minimum deposit is necessary.

Busch Gardens/Old Country, Williamsburg,Va. -- Senior Citizen discounts.

National Parks -- If you are 62 or over you can enter all national parks free. The naturalist at the entrance will give you a lifetime pass.

National Zoo -- Become a FONZ (Friend of the National Zoo) for $10 a couple instead of the usual $15 (single) or $25 (couple).

Swimming Pools. Public pools in D.C. are free for all ages. In Montgomery County where there is a fee, Senior Citizens can swim free. Reduced rate in other counties. Starts at 60.

Education

Many area colleges have generous education plans for Senior Citizens ranging from free enrollment for non-credit courses to a generous discount on credit courses. Age requirements vary.

District of Columbia

Catholic University -- Age 65. Short non-credit course, $10 off.

Georgetown University -- Age 65. 50% discount for auditing courses.

University of D.C. -- Age 65. One-half tuition for non-degree program. District residents only.

Maryland

Montgomery College -- Age 60. For Montgomery County residents. Pay 10% of the tuition. Shorter community-service courses, free.

Prince George's Community College -- Age 60. Free for P.G. residents.

University of Maryland -- Age 60. Free for not over 20 credits. Maryland residents only.

Virginia

George Mason College -- Age 65. Same as No. Va. except for age.

Northern Virginia Community College -- Age 60. No fee if yearly income under $5,000. Virginia residents only.

University of Virginia -- Age 60. 25% discount on non-credit courses. If enrolled the day after semester begins in a class that is still open, may audit free of charge. Virginia residents only.

Elderhostel -- Perhaps the best deal of all. Many colleges in this area are part of the Elderhostel system, but you can attend college for a week or more in many scenic and interesting areas of the U.S., Canada, England and Scandinavia. One week of three or less courses, room and board (you live on campus), all for $150. For a catalog, write Elderhostel, 100 Boylston St., Suite 200, Boston, Mass. 02116. Age 60 and up.