If your diet, training, sleep, and PCT are on point. Yes you can keep your gains even years later. It's been found that after only your first cycle. Muscle nuclei increases, Muscle density can increase for very long periods of time
But if you're still a noob your training will not be on point. Hence experienced users saying you shouldn't jump on a cycle too early, either early in life or early in training career. If you've only been in the gym a year or so you haven't got a clue how to train hard, but if you've done the last 3 months of it on 600mg test a week your going to be over what any natty will achieve in 3 or 4 years. But have no idea how to maintain it.
The way I see it is you will always go back to (near to) the body you would eventually have got with diet/training/rest/lifestyle regime you are currently doing.
To maintain your body at a certain level takes a certain level of training/diet/rest no matter how you get it there. The problem is if you don't get there natty you don't know how to stay there natty.
Changes in numbers of muscle nuclei may help keep gains longer and hold a little more size. If you've got a physical job that will also be a factor in keeping gains without either the drugs or training required to get them.
There may well be a (dodgy USA stylee) level of "TRT" that will keep you there, but as anything over 125mg (at the most, 100mg is more realistic) test per week isn't real TRT that's a moot point, all people on 250mg per week are really doing is living on low cycle, especially if they're on an AI as well, the AI proves they're on a cycle!