Krissy Kendall, PhD, CSCS*D, CISSN, EP-C, and science editor for Bodybuilding.com says, "From age 20 to age 80, there is about a 30% reduction in muscle mass due to a decline in both muscle fiber size and number. Furthermore, there appears to be a preferential loss of type II fibers -- which are the fibers that are most responsive to growth -- with advancing age."
So if the last time you hit the gym was in your 20s, and now you're in your 40s wondering why your body just isn't responding to strength training like it has in the past, well, your age might be playing a role. Your best bet is to keep at it.
Kendall suggests including three to four days of resistance training per week while following a well-balanced diet, "Include ample amounts of vegetables, high-fiber carbohydrates, and lean protein -- you should aim for 1 to 1.2g of protein per kilogram bodyweight. Supplement with vitamin D, vitamin B12, and folate to help preserve muscle mass and strength while aging."