I think the number one motive would be financial stability. And number two more experience is what I've always heard. 

Money, money, money, money and money.

Oh...and because they are more mature, responsible and caring....I think.

Do I count if my gf is 12 years younger?

We call those spring and autumn, or summer and winter relationships.  And I have seen them anywhere from 10 years difference, to almost 60 years !.  A famous one, Cello Master Pablo Casals married a cello student and protegee when he was 83 and she was in her late 20"s (if I remember right). 

It is hard to find out what it is in peoples' minds, but some of the reasons for this are:

1.- Some women do prefer more mature men.  Maybe because mentally, men are slower to mature than women, so in choosing an older one, hopefully they would have the same "mental age".  Or is it the father syndrome ?.

2.- Financial considerations, better quality of life by having access to more disposable income.

3.- Older men face life in a different way that younger ones.  While younger ones still need to fill their sods, older ones are more responsible, value things more in a sentimental relationship, and do care more for the well being of their partners.

4.- Older men are more attentive to the needs and wants of their partners, especially Western men.  Maybe even to the extent of realizing that by giving (with proper safeguards) and making the other half happy, they can also become happier too.

Haven't Chinese traditionally married for a good husband to look after the wife? This is not the same as being a gold digger. Marriages used to be arranged. Along time ago in European culture, it was similar. The Chinese still follow traditions, perhaps because they are still developing. It did work in the past. Older men can have more to offer a woman.