It is easier for both older men and older women to procreate with younger partners (women not yet through menopause obviously) so perhaps there is some biological reason behind it.
I've noticed since my late 30's not only increased sex drive but also increased reaction to/lusting after (!) younger men.
The other thing is that I find men (not all obviously) more inclined to think in a me, me, me - what do I want way than women hence some of them don't do the "but would a younger person want/be comfortable sexually or in a relationship with me" thinking that a woman would. They seem more inclined to objectify the opposite sex.
Anyway - what they want and what they get are 2 v different things (unless they are wealthy of course).