It sounds like you're eating all the right stuff, plenty of protein, etc., but are you doing any strength training (i.e., lifting heavy stuff)? It's not true that you can't add muscle mass after a certain age, and, in fact, it's pretty important to at least keep what you have. If you don't move it, you lose it. (I'm looking at what I just wrote as a critical reminder to myself, too, by the way!) You can get a set of resistance bands pretty cheap online, and with a little ingenuity you can "lift" a lot with them.