When I studied at Lubavitcher Yeshivah (about 30 years ago), I learned a sicha (talk) of the Rebbe on Parshas Va'Yera (Gen. 17).
I believe the sicha appears in one of the earlier books of the Yiddish volumes (aleph - vav)
In general, I am sharing my memory of the teaching with you based on that sicha and my Yeshivah learning at that time on the subject of that Mishnah in Pirkei Avos (5:25).
It appears that you have only a snipet of the entire teaching (the OP's reference to the Rebbe talking about a 100 year old etc.)
I hope by sharing the rest, that it would sound more satisfying.
The Rebbe starts with a famous story about the Tzemach Tzedek (3rd Rebbe of Chabad-Lubavitch) and his grandson the Rebbe Rashab (5th Rebbe).
When the Rashab was a boy of about 5, he came into his grandfather's room crying. The Tzemach Tzedek asked him why he was crying?
The boy said that he just learned in Cheder that Hashem "appeared" (revealed Himself) to Abraham. The boy was so upset, because he also wanted that Hashem should appear to him too!
The Tzemach Tzedek answered: "When a Jew of 99 years of age decides to circumcise himself, he merits that Hashem appears to him." The boy calmed down.
What did the answer mean?
The Rebbe Rayatz (6th Rebbe) explained that a Jew does not recon his years by his passport date. There are older people who are "young" (immature) and so called younger people who are considered old (in wisdom and maturity). Actual age is not the deciding factor; rather it is spiritual development.
The Rebbe then explains that this applies to the entire reading of Pirkei Avos 5:25. (the ages there are from 5 - 100)
The Mishnah is not discussing physical ages, but rather discussing levels of character refinement.
(Kabbalah teaches that the soul has 100 levels of refinement available because there are ten sefirot in the "image of G-d" in a man. In this world of "Tikkun" (repair) the sefirot may unite, thereby producing ten times ten = 100 middot (character) combinations to work on. NOTE: the popular combinations listed in sefiras ha'omer are only 7x7=49 middos, because only the purely emotional combinations of middos are being focused on. (Chesed - Malchus) However, there are really ten including the intellect; being the 3 of Chabad as well.)
We should notice, that the Mishnah says "ben-arbayim" (40) and not "ben-arbayim shannah" (40 years) for instance. (Although the word "shanim" is mentioned at the beginning by the youngest ages, it changes later.)
Although it may be that it takes an average man a year to refine each middah, the truth is that everyone is different and may refine themselves at different paces with some middos taking longer than others.
So, the Mishnah is not binding itself to actual age. Rather, it is a scale of 1-100 where each level is another percent toward 100% mature development of the soul and body in this world.
As each person develops further, they are capable of more levels.
So, the Tzemach Tzedek's answer to the young boy was that when a Jew who has worked on himself tirelessly to achieve 99% development, runs to circumcise himself for Hashem, because 99% just isn't good enough; he must have 100% closeness to Hashem, then Hashem feels obligated to reveal Himself to him. (This explains why the boy would stop crying as obviously he was only 5 (5% developed), and had no reason to be worried that Hashem was ignoring him. He now knew that if he worked on things, he could achieve such revelation.)
Therefore, the "90 year old is bent over" actually means that the "90% master of middos has his body and yetzer hara (evil inclination) bent over completely to his own spiritual control."
The "100 year old is "like dead", null and gone from the world" actually means that the "100% master of his own middos is not subject to physicality at all and it is as if they are not bound by this world."
Pirkei Avos is a set of teachings to improve middos. It is not a simple commentary on aspects of life. So it makes more sense that the message here, is one of human character development, and what someone could achieve if they work on it.