I respect your question, but I am afraid that any answer would be a generalization and therefore not representative of the truth.
The question must be much more specific and the only way to answer it would be to conduct a scientific study. All other opinions are merely opinions that are not based on any statistics, but only on vague value judgments.
Older people - age group?
The most significant contribution - would be a value judgment.
What kind of older people - what status, what social class, what country,
what ... what... what...
Our opinions are so far from precision. Every generation (if there is such a thing as a generation, which is normally the group with the age difference no more than twenty years) has a very distorted views of its own generation let alone the generation that follows (or precedes).
I have once read a quote (I won't be able to find it now) where a man of an older generation described modern youth as being completely impossible - rude, loud, ignorant, disrespectful... and I nodded in agreement. Yes, that rang so true. Then I looked who said it. It was a philosopher from the Ancient Greece.
In order to simplify my reasoning, I would just rephrase your question -
What is the most significant contribution "my generation" made to society?
Well, global warming. Self-destruction. Wars.
Refusal to think in the global terms. Technology. Overpopulation.
I am thinking that maybe every generation contributed to the survival of the species as in reproduction.
Generation is larger than society.
My parents (70) are under the illusion that I will never understand them because of the age difference. As in "I am not there yet". But what does not work in this argument is that I may never understand them, but they must be able to understand me because they have already sailed through my age. But they don't.
This dilemma was bothering me because I did not have an answer. It turned out to be so simple. Our emotional memories deteriorate and we forget how it was to be children. I did forget it, too. I don't understand my son (10). I believe that he will grow ignorant (a generational thing?). But I still love him.
What was the most significant contribution I made to society? I really cannot say. But I would recommend to start pondering this question from the perspective of an individual, rather than such an enormous group. (40)