I felt a decline once I hit 40. I felt I was getting old. In retrospect, it was due to several factors, the main ones being: bad marriage (i.e., no desire, etc.), lack of exercise, and diet. We divorced several years later and I set out to remake myself—to regain much lost self-esteem and to make myself ‘marketable’. I.e., I hadn’t dated in almost 20 years and was definitely feeling out of the loop. To make a long story short, I start hitting the Y almost every day, switched to a highly holistic diet, and—eventually—met the right woman. It’s the combination of all these things that I feel has brought back my libido, with a vengeance. I’m not as hair-triggered as I was in my 20’s, but I certainly feel as sexually ‘strong’ as I did then, with tons of stamina (I can provide references!). Plus, I’m much more relaxed about it, and more experienced. Less focused on orgasm (mine, anyway) and savouring the whole experience, the details, the nuances, the closeness, the passion, the incredible connection between two people in love. The net result is that I’m having the best sex of my life now. Don’t underestimate the power of any these factors, particularly the third. (My gf can get me hard by talking about groceries over the phone :-)
(Now 54, btw.)