I think alot has to do with the hormone levels dropping as we get older. That and other aspects in out lives, as in work loads, social interests and if you have kids their issues have to be dealt with too. This can be very taxing on the mind and body. Think back 10 years ago when you were having sex more regularly, you weren't doing what you are now and probably didn't have kids and or spouses.
Of coarse the gear helps to raise the test levels artificially, but when this is finished and your natural levels are low, of coarse you're going to feel a little less interested.
Lucky us women tend to peak in our 30's. Hmmm, kust have to find some one to use it on, lmao.