Can I still gain muscle if I am over 40?

The maximum contractile ability and power generated by a muscle decrease by 30-40% between the ages of 30 and 80 which makes muscle gain more difficult.. This decrease in tension-generating capacity is due primarily to a decrease in average fiber diameter. Some of the change is simply the result of decreased physical activity with age and can be prevented by exercise programs.
The ability of the muscle to adapt to exercise, however, decreases with age. The same intensity and duration of exercise in an older individual will not produce the same amount of change as in a younger person. This decreased ability to adapt to increased activity is seen in most organs as one age.
This effect of aging is only partial, and there is no question that even in the elderly, exercise can produce significant adaptation and results.