There are different points that need to be touched here.

First, as always, there is a distribution of effects, which means that there is individual variation. We can reasonably assume that, _on average_, the older people are, the less they care about other people's opinions. The age after which these effects are noticeable is not immutable since the dawn of times, but depends on when people have kids, are expected to retire, etc. For example, I expect the age at which one cares less to be later in life now that three decades ago, when people at 40 were already considered way past their prime. I mean, for the generation of my parents, physical activity after they turned 18-20 was for wealthy people or "eccentrics".

Second, it depends very much on what is settled for the person or what is still fluid. If someone has a family or has reached of the "peace of the senses", they are already caring much less about the major source of frustration (much more than source of joy) and status-seeking behavior for most people: sex. If somebody has also given up on practicing sports (or any king of competitive hobby), put up the classic 40 pounds more at 40 years old, they care even less about others. Lastly, hormones change, testosterone in men visibly drops, women start getting into menopause. And then it depends on personality (see the point above about variation), there are people who are still combative in the status game at 80 and others who have given up at 21.

In synthesis, it is not that older means wiser and thus the "who cares what other people think" comes from a place of introspection (between us: sex is incredibly pleasurable), but more simply that hormonal changes, "giving up" (sex, competition) put people, sooner or later, on the bench of life: spectators, more than players.

> there is a distribution of effects, which means that there is individual variation.

You're saying this to indicate that anyone who disagrees with you is an outlier, but what you have claimed is completely un-relatable to me and I don't recognise it in (most) other people that I interact with.

Giving up on sport/fitness and normalised obesity is a uniquely modern phenomenon, and almost uniquely American.

Giving up on sex and competition is not something you can really accuse the Boomer generation of. You only need to look at the demographics of the US Presidential candidates, or any other elite career in 2020, to understand that status-seeking is most pronounced in that generation of men.

I don't remember being this misunderstood in recent times, so let me clarify before moving on (and I don't think dismissive comments like the one I am answering to provide much benefit to the conversation).

"You're saying this to indicate that anyone who disagrees with you is an outlier" - Not at all. People who disagree with me have a different view on the issue, different experiences. That there is variation is inevitable in every complex, non-deterministic process (like status-seeking behavior, feeling at ease with oneself). On the contrary, complex, but deterministic (like how many hands one has), have very little variation.

"Giving up on sport/fitness and normalised obesity is a uniquely modern phenomenon, and almost uniquely American." - I disagree. And I am not American, although I have been living in the US for years; I am from one the countries who are considered "healthy". We need to distinguish differences between cohorts (over time) and differences within cohorts over time, but I don't want to make this too long (interesting pub here that supports what I had written: https://www.tandfonline.com/doi/full/10.1080/17461391.2019.1...). Up to the 80s, adult sports for hobby basically did not exist outside of the wealthy and the (yes, in this case) outliers. There are data and anecdotes, let me use the latter here. Among my grandparents' social circle (the vast majority not being among us any more), there is not a single one who did any sports after they were 18-20. So we might not even see a drop in activity, because the activity never started. In my parents', there were a few here and there, some who played tennis (well off people), some runners, some playing golf, the boxer, the judo guy (for women, outside of maybe some dancing, zero sports I remember about). When I started training with weights in the early 90s, gyms for adults were far and few in between. Now, in 2020, there are more gyms and activities for adults than ever and participation in adult sports activities has never been higher (see publication above). Apart from the sick and the exception, when I look at photos of my parents and grandparents' friends (social media, old weddings), they are all fatter than they were in their 20s. In my home country, up to the 70s, calories were simply not abundant and/or not convenient (if you need to start from raw potatoes to have french fries, you are in for the long haul).

"Giving up on sex and competition is not something you can really accuse the Boomer generation of." - I have no idea where this is coming from, surely not from me. But if you ask, across every generation, the distribution of sex-seeking and status-seeking behavior shifts (and it is surprising I have to write this) with the age in the direction of less sex and less competition (related to Boomers, one hypothesis is that the variance increases with age. It might be, it should be tested). Now, if you ask me to make a prediction (which will be proven right or wrong, we'll see), for millenials the shift with age will be slower, more will use TRT, more will have kids later in life, more will feel healthy and competitive (not competitive at the neighborhood meeting) later in life. And yes, we read about the horny boomers, but rest assured they were much more interested in sex when they were in their 20s. Hormones are powerful! Then, if you and your social circles had different experiences and you are all having more sex at 50 than at 20 and you are more competitive in sports now that in high school, I am fine with it.