A common theme I see with these articles is how "grueling" the work is. People need some perspective on what grueling work is. I worked at a UPS hub during college, where I'd load, unload, and sort the contents of trailers. What they do seems pretty easy compared to what I did at UPS. Picking and packing orders is a lot less grueling than unloading trailers full of 60 pound paper boxes, or loading hundreds of package…
I think a major difference you're overlooking is that in your late teens and early 20s, you can do pretty hard labor without much effect.
I'm only in my mid 30s now and when I do a hard day's physical labor, I feel it a lot more.
Someone in their 40s or 50s or 60s is going to have a much harder time doing that kind of stuff every day.