Some things in life are harder in your 40’s than they are in your 20’s. Building muscle and maintaining a healthy physique isn’t, however.
It is entirely possible to continue (or begin) to build lean muscle mass in your 40’s and beyond. Indeed, your approach doesn’t need to change radically and the key fundamentals remain the same
This is the key takeaway, the processes aren’t any different, it’s just that your margins for error can become somewhat narrower.
This means it’s important that every element of your training, nutrition and recovery are on point so you avoid the niggles and injuries that are more common in your middle years, and unfortunately often more debilitating.
Focus less on the 1 rep max and comparing yourself to others and more on perfect technique, controlled lifting, solid nutrition and all over body mobility. Do this and your muscle building journey can continue well into pensionable age and beyond.