I was bigger and stronger in my 20s, my third decade of life, completely natural, benched 315 four times, but I am in my fifth decade of life, and I have not been able to get back to that level of strength (stuck at 275 for 5 reps or so). I am guessing it has to do with age and a decrease in test. Starting a cycle caused me to shoot past where I had been stuck naturally for years in just a couple of weeks. This seems to confirm that a man in his forties does not have as much test as a man in his 20s. I would say I was at a natural limit - limit that was less than what my natural limit was previously. This changes throughout your life. A 25 year old guy is going to be bigger and stronger in his late 20s, even if he thinks he is at his natural limit at 25. And he might even get stronger into his 30s, and at some point he will start tapering off.
Yes, it will happen to you, too. You will turn into the guy who is in great shape "for his age." LOL!