Well, gentlemen, I can see that I will have to be the Dutch uncle of this thread. I do not consider myself an expert in romantic relationships, but I do have experience with women both older (by 17 years) and younger (by 20 years).
Age differences in a romantic coupling DO matter, if the intent of the couple is a life-long commitment. If the intent is just to have fun and enjoy each other's company, age differences matter much less. Both men and women experience somewhat predictable changes as they age: we are always in the process of becoming someone else. These changes are more pronounced in the younger years than in the older ones: a 20-year-old changes more in the five years to age 25 than does a 40-year-old in the five years to 45. Our likes and dislikes, our concerns and motivations, our attitudes and priorities all change greatly when we are younger, due to the unique background and experiences we each have in our lives. The more changes that occur in one or both of the partners, the less stability will will be present in the relationship. Thus, though age differences always matter, they matter less if both partners are older than 25-30.
Ther are other concerns: communication can be challenging if both partners do not have the same cultural interests, shared activities can be curtailed if they are not each at a similar level of physical fitness, activites with friends and families can be awkward and uncomfortable and introduce great stress into the relationship. The presence or absence of children in a marriage has already been mentioned. In addition, the greater the age difference, the less the relationship will be perceived by one or both parties as a partnership of equals: this is guaranteed to introduce stress and instability.
Those who enter into a long-term relationship with someone much older or younger must consider all these things if they want to build a successful partnership. In general, the older both parties are, the higher the degree of success can be expected, since our self-image and maturity usually become more stable as we get older.