When you’re 23, in 20 years you’re going to be 43. Most folks say that’s a trillion years away. But when you’re 63, in 20 years you might not be.

So, with Elul here, you should get selective about how you spend your time.

All of us have heard that people wonder how much they would change if they could live their lives backward, acquiring at life’s beginning the lessons they had learned at its end. Another Elul tip: how greatly our lives would be enriched if we were able to imbibe not just Pirkei Avot (Ethics of the Fathers), as we do this time of the year, but also Pirkei Savim, the wisdom of our bubbes and zaydes. They could offer us quite a bit.

From American lore: When the noted judge Oliver Wendell Holmes was still active on the U.S. Supreme Court, he and Justice Louis Brandeis would take walks every afternoon. On one of these occasions, Holmes, then 92, paused to look with real admiration at a beautiful young girl who passed them.

They stopped, and the elder said to the younger with a sigh, “Louis, oh to be 10 years younger again.”

Halfway between my 40th and 50th birthdays, a big change came over me. For the first time in my life, I was confronted by the thought that my years past almost certainly outnumbered the years ahead.

At some time in our lives, we all must face this. What can we do to lessen that shock?

We can write — be it diaries or letters or articles — without even using a pencil or pen in this computer era.

We can record our thoughts by working out the questions that we can answer, or by having someone else, our children or oral history professionals, quiz us for the record — for the benefit of the future, of course. At this time when amazing inventions are bought for as much as a billion dollars, we can sit down, surround ourselves with talented people and invent.

This is not for the money but for everlasting name recognition.

One of my uncles, Abraham Geffen, a professor in the field of radiology, invented the Geffen Ruler to measure certain organ movements on an X-ray.

Who knows? The Berman Mouth Bite, the Cohen Internet Clipper or the Levy Cell Locket could come from your brain and your hands.

I learned this from a Talmudic sage one summer at Camp B’nai B’rith. In a lecture he said emphatically, “A midlife crisis is merely G-d’s way of making us ask ourselves if we are living to our full potential, of making us take the responsibility for that within which remains unlived.”

Now that I am in the middle of my eighth decade, I hope that I can honestly say I experienced the joy of passing on hochma (wisdom) to younger people.

A person I knew well but had not seen or heard from for 20 years wrote and brightened my day. “David, every week or so I say to someone in my bakery shop, ‘David taught me how to treat people this way.’ ”

I never realized what I had done, but now I have at least one point in my favor — sure that through Elul, you also can record all the good points you have accumulated. Don’t just repent in the days ahead; calculate all the positive stuff with which you have filled your days.

“A midlife crisis is merely G-d’s way of making us ask ourselves if we are living to our full potential.”

How many of us have read Shakespeare lately? In Jerusalem, we can now watch “Shakespeare’s Plays on the Run.” We do the moving from site to site in different parts of the city, and the actors and actresses await us. It is almost like waiting for Godot.

In “As You Like It,” Shakespeare divides life into seven stages. What a great time of the year to read the insights of this master.

Man/woman begins as “the infant mewling and puking,” then becomes “the whining school-boy, with his satchel and shining morning face, creeping like a snail, unwilling to go to school.” Later we become the lover “sighing like a furnace;” then the soldier “seeking the bubble reputation even in the cannon’s mouth.”

At the fifth stage, we are “the justice in fair round belly with good capon lined.” Moving on to the next stage, the individual is seen “with the spectacles on nose and pouch on side.”

William Shakespeare pulls no punches as he tells us we will end with “second childishness — sans teeth, sans eyes, sans taste, sans everything.”

The question is, just as we squeeze notes into the Kotel, how can we make sure to focus on the spaces in our lives, waiting there anxiously just for us to take an interest so they too can be filled.

My friends, you either rust with disuse or grow musty with stagnation. But it can be different.

If you have the feeling that there is something special you are endowed with — yes, you have a talent that can be used well, grabbing hold of yourself — giving it a try (even if you do not succeed) will add a dimension to your life you never expected.

Dor holech, dor ba — a generation comes, a generation goes,” Ecclesiastes informs us. What this means is that each of us is a significant link in the chain of generations.

A wonderful function of the computer is that its innards can reach out to the past and pinpoint those ancestors of ours. We may never have seen their faces, but when we truly know that they existed and that is why we are here, our personal being can take on new meaning.

Do we have to spend hours digging deeply and becoming aware of the circle of life they led? Yes, if we so choose, as genealogists daily make discoveries of human beings and not clay pottery.

Each morning of Elul, tiku bashofar (the sounding of the shofar) proudly says to each of us, “You are here. They were here. Your children will be here.”

Now it’s your turn to grab what you have, both character and money, and leave an inheritance, whatever it may be, to shape the future. Am Yisrael chai now and for eons to come through us.