There are lots of factors. At a certain point, usually after 50, your serume testosterone level drops and you go through what is now called andropause (male menopause). With a reduced T level, your libido drops and so does the ability to have or sustain an erection.
Genes play a part in exactly when this happens and testoserone therapy (like gel patches) have certainly proved to help in that department.
Your level of sexual activity through your life also plays a part, and most people, men and women, who have been sexually active all their lives tend to stay that way.
Blood flow is important as well so anything that restricts this will restrict you. Among the factors are diabetes, coronary artery disease, the drugs you take daily (some reduce libido).
Your own self image is also involved. Sometimes as we age we don't really like what our bodies have done, and that can translate into not being able to perform.
And you are not 20 any more as previously said. Getting there will take a little longer and have patience. The best things in life are worth waiting for.
And stop watching the pharmaceutical ads because they only want to convince you that you have something wrong in order to sell you a product. ED happens in one way or other at some time to all guys...wrong time, wrong mood, stress, etc. It's not a unversal dysfunction. It's your body telling you something's gotta change.