When you fall in love, the age of the person is the last thing on the mind, you are in love and that's all that matters. There are many lucky couples who experience years of marital bliss despite differences in age but for others the age gap starts to affect the marriage as the years pass. The younger partner starts to feel unfulfilled and unhappy when their expectations of marriage are not met.
The older man (and by this I am assuming the man is at least 10 years or older than the woman), might not have the drive that the woman expects of him. If he is not particularly fit and active, the difference in years will show up at some stage or another when he wants to take things slower and is not as active physically as a younger man. In the beginning, he might make efforts to keep up with his younger partner but over the years the efforts might wane and this is when disillusionment sets in. There are obviously older men who might be fitter than their younger partners and these are exceptions to the rule. They can live up to the expectations of a woman much younger than them and a few famous men come to mind who are seen with women nearly 20 years their junior.
These are the men people look at with a raised eyebrow because the first thing that comes to mind is money. Why would a young beautiful woman be with a man old enough to be her father? It seems unnatural and again in a few cases it might be true love, but more often than not, the man is rich and that is what the younger woman is attracted to. All her material needs are met and for some women that is the priority of their lives. They have a house and security and they are happy, age is immaterial in this case. Others might want a father figure for their children from a previous marriage and they are happy if an older man can provide this love and security. Again the age difference provides no obstacles for them; it is the best way forward at that point in their lives.
In some cases, the wisdom and experience of an older man is extremely attractive, particularly to a younger woman. If she was a daddy's girl and very much loved by her father than it is difficult sometimes to find this quality in a young man her own age. It is the older man who will give her the same sort of stability, love and support that she has grown up with. It will not be hard for her to fall in love with such a man reminding us of some aspects of Freud's Oedipus complex.
While financial stability seems to be the primary reason why a young woman would want to marry an older man, there are some women who fall in love without any pre-planning. They cannot help who they fall for and it does not matter if the man is 10 or 15 years their senior. Love is blind as they say and they have not planned who they are going to be with. In such cases, love is the reason for their choice and it is definitely the right foundation to build their married life on.