Why muscle gain slows the longer you train
The genetic ceiling describes the upper limit of muscle a person can build naturally, set largely by inherited factors such as limb proportions, muscle belly lengths, hormone profiles, and the number of muscle fibers and satellite cells. Plotting drug-free muscular potential against years of training produces a curve that rises quickly in the first year or two and then bends toward a flattening asymptote as the lifter approaches that individual limit.
The fastest gains happen for beginners. As training age increases, each additional bit of muscle requires more time and effort, which is why advanced lifters progress in small increments.
The ceiling is individual, not a single number
There is no universal endpoint that applies to everyone. Estimation models exist that relate a frame's height and bone structure to a plausible drug-free maximum, but these are population approximations with wide individual variation, not precise personal predictions. Genetics influence both the height of the ceiling and how fast someone approaches it.
This is general educational information about a physiological limit, not a personal forecast or medical advice; realistic expectations are best set with a qualified coach over time.