That it takes decades to reach your so-called natural limit in muscle mass is just so much bullshit. The absolute majority of muscle you're ever going to build is built in the first couple of years of intelligent lifting and eating. Let's say something like at least 90% in the first 2 years. The next 2 or so years you gain the next 8-9% and you're basically THERE. For all intents and purposes you're for sure finished growing after less than 5 years of reasonably intelligent lifting. What a few posters have said here, that they're not really growing much after decades is true. "Muscle quality" may improve, though that's pretty vague and subjective term. Muscle growth isn't a slow process where you add a steady 1-2lbs a year for decades. It comes in bursts and drug free maximum muscle potential is realized very quickly.
I know people who have lifted clean for over 20 years and claim they're still growing and "progressing". I wonder how they measure this "growth" since they've weighed the exact same for 2 decades, and in many cases they're actually lighter and weaker than they were in their 20s for example.