Age Gap or Gap of Ages? (Relationship)

Recently, I've been thinking about why men choose a much younger woman or vice versa: why women choose a much younger man. The age difference of which I speak is more than 10 years, sometimes even 20 or more.
What do you think, what could be the reason? What is your country like and how acceptable is it in society? Is it just because you cannot find a partner of his/her age or ..?
Dealing with this phenomenon, as I saw, varies widely, and even though we make our own choices, it's rare that the opinions of close family members or friends would leave us cold regarding this point.
Thanks for your thoughts?