Guys over 50 or so are just different, psychologically as well as physiologically. It becomes much more of a mental thing, and that's where the arousal/erection occurs.
The problem is that women become so accustomed to men during the first part of their life, where they often want to fuck anything and everything and a stiff breeze causes an erection (possibly how it came to be called that?)
This seems to be a change in perspective that many women have difficulty grasping and adapting to. That is unfortunate, because what it really signifies is that the man is attracted to them personally and individually, rather than a mere universal desire to hump that so often dominates male sexuality earlier in life. This may also explain why those women who "get it" are attracted to older men, although obviously there may also be other factors at play there.