Some of the words here make me feel so young at 55 . . . even though I'm older than almost everyone at three of the milongas I attend. Of course, I'm younger than just about everyone at three others, and about average for all the rest.
There are definitely different factors at play when it comes to aging and tango:
Acceptance. However long you've danced, however wise you become about tango, however much respect you feel you've earned . . . once you become "the old man" or "the old woman," people who are younger, fitter, and closer to their lessons won't invite you as often. While dancing at the milonga for a long time is educational, it's at lessons that we're reminded of errors to fix and given interesting new ideas to explore. Once the people who are on the sharp edge of their tango learning stop dancing with us, we're dancing with peers who have vaguer memories and more deeply entrenched bad habits, and therefore dancing less well than we might desire. Even at 55, I'm feeling this despite working hard with a coach – at those milongas where I'm older than just about everyone, my cabeceos might as well be aimed at the coatrack.
Aesthetics. Part of the reason why people who are younger, fitter, and closer to their lessons won't invite you as often is that you don't look like somebody they want to dance with and/or be seen dancing with. It's easy to dismiss "I'll dance sexy or not at all!" as shallow, but hey, cabeceo/mirada is by definition about looking at someone. If you're starting to sag, wrinkle, and gray, and have to wear clothing and especially shoes designed to offer extra support to an aging body, you're going to be looked at less often. Where I disagree is with suggestions that this is something that happens to women but not men. It's possible that my "Silver Fox" card was lost in the mail, but I think part of why my cabeceos go straight to coatrack is that I'm not one of the younger, more vigorous guys who get 95% of the tandas with the more advanced followers.
Athleticism. The other part of why people who are younger, fitter, and closer to their lessons won't invite you as often is that age bring a certain inevitable loss of balance, flexibility, speed, strength, etc. It's less fun to dance with you because you can do less. Of course we can work out, eat well, take vitamins, see a physiotherapist, and otherwise do our best to retain our capabilities . . . but we can't undo the majority of the cellular and molecular damage that occurs over time. Tango itself can damage parts of you this way; too much tango can actually make you less able to dance tango.
Energy. A key attribute that declines with age is endurance, stamina, energy. Even if you can keep it high for the duration of one evening, you can't go to as many milongas in a week and probably can't stay as late as you'd like. Lately, with long COVID and the strain of having to move house (another story, but blame my landlord), I'm unable to "cheat" by taking naps and conserving energy throughout the day to function as well for as long at 55 as I did at 25 or 35 or even 45. It'll probably be worse at 65 and 75. And if you aren't at the milonga, or present for the final hour when the music gets cooler and there's more space to dance to it, you aren't visible to those who might dance with you, which feeds back into the not getting invited as often.
Expense. This varies a lot with where you live, but in many places – my area included – retired people are often living on fixed incomes. Sure, some salted away a lifetime of savings, got lucky in the market, and are living their best lives. The majority are not; they're dealing with having less than ever. When you have less money, you can afford fewer events, lessons, pairs of shoes, and all the rest. Tango can price itself out of the market, and even if it doesn't, the lack of money feeds back into getting more distant from lessons, dressing less well and looking less interesting, and going out and being seen less often. As a person who came from no money, had modest jobs, lost his investments in a divorce, etc., this is a serious fear for me. I think I'm more likely to be unable to afford tango than unable to dance tango.