I'm in my 40's, and what I've been noticing is that the muscles that are starting to lose mass, despite still being strong, are my Quads and Hammies. They just don't pump up like they did when I was in my 30's or 20's. Getting older sucks. Been doing 20 rep squats with 225, and that's got my lungs working, but the pump isn't there like it used to be. Lower back not like it used to be, so don't want to go heavier than that.