parazapper
There are several things to consider here and I will mention a couple.
You are aging and there are several associated changes.
DHEA is a hormone that starts to decrease when you are about 30 and testosterone is made from DHEA as are over 50 other hormones. Do not just indiscriminately take DHEA, have your blood tested for DHEA-S and for Free testosterone and SHBG ( Sex hormone binding globulin ). Have a wellness doctor help you to calculate the optimum levels. A regular doctor will not do as they are generally ignorant of the requirements for optimum health. What you want is optimum, not just some number that is within the "normal" range.
Another factor is the reduction of endothelial function ( hardening of arteries, etc. ) that interferes with the normal penile hardening because blood pressure to the penis is not as well controlled.
Important, this can also be an early warning for potential cardiac problems and taking v!agra or Cialis will not fix this.
My recommendation is for you to seek out a wellness doctor. They are hard to find but they will make a tremendous difference in your life. They are expensive because they are more thorough in their tests.