Wisdom and Experience
Older men may be seen by women as not necessarily more intelligent, but perhaps wiser and worldlier. These men are more experienced in many areas of life, having been alive for several decades, and are thus more likely to apply common sense to situations in which younger men might be irritatingly rash. Older men will have picked up more knowledge and likely have many stories to tell, which appeals to women who enjoy interesting conversations and debates.
Not every older man will be rich, but it seems likely to many younger women that a man who has been around a while will at least have acquired a steady job and will live in comfort. An older man is more likely to have his own house, for example, where a man in his 20s might still live with his parents or share a place with friends. This sense of a comfortable life, with the reliability and security that go with it, can be appealing to a woman.
It may be something of a myth, but the assumption many young women carry is that an older man is going to be a gentleman, and possibly a romantic, too. He'll take her out for lunch, buy her regular gifts and always pay his way. An older man is more likely to display traditional, but often welcome, manners, and will hold open a door or pull out a chair to seat his lady. Regardless of whether this sense of chivalry is borne out by all older men, the appeal of the gentleman is deep-rooted within many young women.
Women Mature Faster
In general, women tend to mature faster than men, which is why young guys aren't always so appealing to women of the same age. Younger men may seem immature, preferring to party than settle down, for example. In contrast, an older man will definitely have had plenty of time to mature, and is more likely to match the maturity level of the woman.
Some women identify an older man as a sort of father figure: someone who will mentor them and provide plenty of advice, wisdom and comfort. This may be the case especially for women who have lost their father, or who didn't had a father figure in their lives.