That's simply untrue, and I aslo know of many in said class who have personally told me that their bodies are beginning to give out and they're looking for other things to do.
There are probably SOME trades that are not SUPER physically demanding ... such as to be an electrician, though that can vary as well, but most of the others are physically very tough jobs, from plumbing to carpentry, etc. Yes, you can still do it to a point, but it's simply inevitable that you SLOW down, you start getting back or knee problems. Good luck trying to do one of those jobs if you've thrown your back out. AND you're much less vigorous and your abilities are diminished. Why do you think that professional athletes are considered washed up in their early 40s? And these are men whose very lives revolve around working out and keeping in shape, even in the various roles that are less demanding, like you play baseball as a DH or play first base, where you don't have to exert yourself all that much, or as a catcher. After their early 40s or so, their reflexes start to go, as do their batting averages, as perhaps their eyesite begins to fade, and so on and so forth. That's just a fact. So, just like with programming, NOBODY is going to want a guy who's closing in on 60 compared to taking someone in his 20s or even 30s. After decades of experience, you could be a supervisor, a mentor to others, etc. ... but you'd have diminished role in the actual physical aspects of the job, and somebody just getting into those trades when going on 60 could never assume that type of role, where he'd be the wise sage that others would look up to and consult with. You'd be a newbie at 60. They hire newbies when they're young precisely to utlize their youth, energy, and their vigor ... but then as those attributes and qualities diminish, their experience and knowledge increase. 60-year-old newbie is pretty much a non-starter.
I'm at a loss as to what track you're on. It is absolutely NOT true that people used to retire at 55. When the Social Security Act passed in 1935 a retirement age of 65 was set, and its has been increasing as people live longer. Before that most kept working or had to be supported by their families, simply historical fact. It would be interesting to see what source you have for that fantasy.
As for it being difficult to find work as one ages, that is true. I left a dairy farm position at age 66, at least five years earlier than I had planned, as I needed to move closer to help with my mother who was in her late 90's. I was determined not to draw social security nor from retirement savings until age 70, and finding adequate work was tough, the market place prefers younger. I was fortunate in that I was staying at mom's farm, so my housing costs were minimal.
It is true that some trades can extract a physical toll, but there is a lot of variability. A machinist or welder in a shop won't have the same demands as a welder working on an oil or gas pipeline in the Dakota's in the winter. A cabinet maker won't have the same demands as a house framer. Genetics comes into play, as well as how well one takes care of themselves. My health insurance pays for a gym membership and I need to work more intentionally now (at age 74) to maintain flexibility, especially with some arthritis setting in, but I'm still capable of working cattle. I'm not as quick as I used to be but a lifetime of experience often gives me an edge over the yung'uns. I now work part time at a funeral home and two of us often need to load a 300 + pound casket into the hearse.
You said a lot of the same things I've said, and I never said it would be easy to find trade work as one ages, its not! But what does that have to do with retiring as age 55?
I would though encourage young people to look into the trades (metal work, carpentry, electrical, plumbing, HVAC, mechanics). While traditional manufacturing has been diminishing because of automation and outsourcing, repair work and on site fabrication have growing demand. New technologies and better understanding of ergonomics and kinesiology make some jobs less physically demanding or destructive of the body than they used to be. If one takes care of themselves (exercise, stretching, keep weight under control) most people can work the trades into their 70's while putting aside savings for their elder years.. And yes, they often will move into less physically demanding supervisory positions because of their experience.