Exactly how old is "old?" At what precise spot on the calendar does youth end and old age begin? We've all seen automobiles of "vintage" dates whose owners give them such careful, loving treatment (far more careful, in most instances, than they give their own bodies) that these conscientiously shined-up old buses perk along merrily for years. While it's true that these venerable autos don't have quite the same dash and class as today's sleek models, yet, ironically enough, there are not infrequent occasions when these antique models are seen blithely chugging past their streamlined and stalled mechanical descendants.

Then we all know the fellow who buys a handsome new car - all bright paint and shiny chrome - and has it looking and running like a wreck in six months.

So there you have it - one car still in service and going strong despite the many, many years of license plates that have adorned its out-of-date chassis; and the other car, not a year out of the factory, ready for the junk yard. The same thing holds true for the human machine. The sluggish, half-alive person who moves through each day with no more effort than absolutely necessary, for all his scant twenty, thirty or forty years on this planet, is literally older than the peppy man or woman past forty who wakes up each day with a genuine interest in what life is to hold for him or her in the next 24 hours.

Have you ever given any serious thought to the difference between living and being alive? That section of living tissue from a chicken heart kept for a number of years in a jar by the famous Dr. Alexis Carrel was not alive - it did not run about the barnyard, enjoying to the fullest its God-given faculties as a chicken. Likewise, the man in a coma lives, but he is not alive.

Staying young means living with vitality, in a body surging with power and energy, and with a mind tuned in to the vital principles of happiness.

If you're not alive to all the powers that are yours as a living, thinking human being, then something is wrong with the chemistry of your body. You probably know several persons of your same age who are far more alive and youthful than you are.

The psychologist would probably say that this difference in the ability of two persons to remain keenly alive to their surroundings and to their possibilities is a matter of "individual temperament." But I say that it's primarily a matter of body chemistry.

So-called "temperament" is chiefly a question of how well or how poorly your endocrine glands function. And the efficiency with which your endocrine glands function - or do not function - depends almost wholly on the food chemicals you provide for their intricately vital work.

That is why the sluggish person, of whatever age, who moves through each day with about as much energy and vitality as a snail in its shell, is probably killing his own pep every time he opens his mouth to drink or eat.

"Getting old" is not a matter of calendar years any more than growing older presupposes becoming senile. Lead your body through the ages with a strong, young and healthy mind.