Because, as one grows old, the heart gets weaker, the blood
vessels are not in as good a condition as before.
Therefore, an old man might find that when he needs blood to be
pumped to his penile region to sustain an erection, the heart
cannot pump the blood fast and furious enough, thus resulting in a
failure to attain a Grade 4 Erection.
On top of that, an old man might have pre-existing health
conditions that might hinder him getting a Grade 4 Erection
too.
Satisfied with the answer? 
