Managing Your Money : RETIREMENT : The Long Haul : Forget three score and ten. Save as if you’ll live to 100. You may.

No fanfare greeted the moment, but sometime in July, 1983, the number of Americans over age 65 surpassed the number of teen-agers, and the United States changed from a nation of youths to a nation of elders.

It’s not your imagination. We are aging fast. The over-85 group is the quickest-growing segment of the population and could more than quintuple to 20 million by the year 2050. By 2080, Willard Scott’s “Today Show� successors could be wishing happy birthday to 5 million U.S. centenarians.

Thanks to biomedical progress and healthier lifestyles, people are living longer. If you’re just a little lucky, you will too.

That’s the good news.

The bad news? You’ll have to support yourself for three full decades after retirement--a dozen years longer than retirees today.

“People have to be aware that they may outlive their resources,� says Don Underwood, vice president of retirement plans and services at Merrill Lynch.

Worse yet, most people already underestimate how long they will live and how much money they’ll need in their later years.

According to a Merrill Lynch poll, people in their 30s are increasingly beginning to see that they--not their employers or Uncle Sam--are responsible for ensuring that there will be a nest egg to tap in their final years. The only problem is that many of these same people aren’t setting aside any more for retirement than in the past.

Fortunately, it’s not too late. You can plan for really old age. It will mean sacrifices in your middle years, but it will be worth it. The important thing is to start as young as possible and to be disciplined enough to keep investing on a regular basis.

“A lot of people, especially those not in the older population now, will have to think about self-directed savings and be much more conscientious about their own involvement,� said Bob Harootyan, director of forecasting at the American Assn. of Retired Persons.

Paternalistic private pension plans, many of which were raided or doomed by the debt-crazed 1980s, are becoming rare. Companies working to cut costs are turning more of the investment burden over to workers, who must decide how to manage their own 401(k) or other plan. By one estimate, only 46% of U.S. workers have an employer plan at all.

What about Social Security? The system has surpluses now, but as the ratio of workers to retirees shrinks in coming decades, those surpluses will evaporate. Moreover, the age at which Americans become eligible for benefits is rising--to 67 from 65. And remember: Social Security was always intended only as a supplement.

As investment experts see it, the key is not to become too conservative too soon by cashing in growth stocks or mutual funds and turning to bonds for fixed income. Inflation will get you if you do.

“Especially if you’re going to live to the ripe old age of 90 or 100, you really need growth,� says Barbara Brandell, a vice president and San Francisco branch manager for Fidelity Investments.

Historically, she says, that means staying with equities, which have outperformed certificates of deposit, Treasury bills and corporate bonds over any 10-year period since the 1940s. Workers in their 50s and 60s will have to resist the urge to shift completely out of their riskier investments.

“Time is a friend,� Brandell says of stock market investing. “Stick with it.�

At Merrill Lynch, the recommendation for a “balanced� portfolio is 55% in stocks, 40% bonds and 5% cash. For the aggressive customer, Underwood adds, the suggested mix is 75% stocks, 20% bonds and 5% cash.

The shift toward longer life has brought a fundamental change: Retirees are no longer staying retired. “We used to view retirement planning as a three-legged stool--Social Security, pension and an individual’s investments,� says Kay Wright Hardy, AARP senior program specialist. “Now, it’s a four-legged chair that includes employment.�

That employment will often be part-time, and a driving force will be retirees’ fear that they will be unable to cover health care costs.