''It's not how old you grow but how you grow old.''
This is the simple wisdom given new urgency by the world trend toward more and more elderly people in relation to the younger population.
In this wisdom lies the key to what individuals and nations can do to make the long-sught goal of increased longevity the boon it was meant to be - instead of the problem being seen in recent national and international forums.
This fall the United Nations General Assembly is supposed to take up the report of the UN's recent Vienna conference on aging. In the UN's host country there could be at the same time a special congressional session to act on the forthcoming report of President Reagan's national commission on social security.
At least, such a postelection session has been called for by Senator Dole, chairman of the Senate Finance Committee. And it would constitute a mark of appropriate concern for elderly people when even the richest country is facing a budgetary crunch in meeting their needs.
This concern has already been manifested in last year's White House Conference on Aging, the current administration and congresssional efforts to eliminate forced retirement, and a new foundation being set up to offset the relative neglect of the aged by private philanthropy. PossiblyPztyh /tions America devises could offer examples or at least encouragement to the third-world countries whose coming challenges were so soberly depicted in Vienna.
For how people grow old can be affected not only by their own attitudes and actions but by the societies surrounding them. The Vienna conference was reminded that the current ''teenage boom'' in the developing countries is projected to result in a swelling proportion of elderly in the next century. By the year 2025 there are expected to be more than a billion people over the age of 60, 110 million over 80.
In the United States, people over 65 are estimated to comprise 11 percent of the population, rising to more than 20 percent in the next half century.
Too often this whole prospect is narrowly regarded in terms of the burden placed on younger, working generations to support their elders. Fortunately it is being increasingly seen in terms of the needed contributions the elderly can continue to make as their own active lives lengthen. In the US social security system, for example, the tax drain on younger workers could be alleviated with a greater proportion of older workers continuing to pay payroll taxes and postpone benefits.
The Vienna proposals would help to maintain and utilize the abilities of the elderly through such means as reducing poverty, checking crime, and getting rid of age discrimination in labor markets. The new US foundation, sponsored by Philippe Villers of Automatix Inc., would reportedly work to reform governmental policymaking on the aged - in part through scrutinizing and combatting ''deep-rooted discriminatory attitudes and practices that limit opportunities for elderly people.''
Here is where individuals come in again - not how old they grow but how they grow old. For the elderly can be tempted to see their own selves with the biases that limit opportunity: the easy pleading of ''too old'' instead of the quick rejoinder of the Roman sage who began studying geometry very late in life. He was asked if now was really the time for him to be learning. He replied: ''If it is not, when will it be?''