I know that people above 30 are still able to gain muscle mass--all other things being equal--but my speculation is that it slows down. That is, that rate of improvement compared to someone in their 20s will be slower, and it will be even slower in your 40s, etc., until you reach a point where you can no longer gain enough strength to offset your losses from age.

I also suspect that the use of illegal anabolic/androgenic steroids could counteract that, but at the cost of other, significant health risks (heart disease, stroke, liver damage, etc.).