Whelp... At the risk of turning this into a huge /hm/ discussion, over the years "twinks" have become a lot softer. It's something of a shock to flip through old gay porno mags from the 80s.
Young, smooth, skinny guys in their early-mid 20s had a toughness to them they don't have anymore. It's especially obvious when you go back another decade to the 70s when "twinks" were still called "chickens."
I gave it some thought over the years and I'm fairly certain I figured it out. Back then, young men spent a lot more time outside doing manual labor from an early age. I don't just mean rural farm work, but even in the cities, they usually earned money doing hard, crummy jobs getting paid off the books because they were way underage.
All of that affected how they developed physically. It isn't like now, where younger guys go to the gym and work out. Gym bodies are very different than the body of guys who never went to the gym in their lives but got up every day at dawn and put fuck-knows how many miles on his bike doing a newspaper route before school since they were 11. Add an part-time/ after-school/ weekend job, and it put on a different kind of muscle.
Guys had much more physical hobbies back then, too. They went outside, played sports, fixed up motorcycles and run-down cars, looked for trouble, stole shit, that kind of thing. Nobody spent day after day playing video games or hours on YouTube/ 4chan/ FB/ or whatever, because those things didn't exist.