Shri SV, you sorta have the knack of taking the words out of my mouth... :becky: what I wanted to say, you have said better...
Anyway here goes...
Biological tunings over thousands of years have happened and hence the ritual, custom or norm or practice in one geographical area may be different from the other. Today, the world has shrunk and cultures have mixed. Such a mixture has both positives and negatives... positives, in that we learn the good from the other, enjoy variations, look through the eyes of different people and broaden our horizon of knowledge.
On the other hand, this also leads one to regress one's thinking to equate with the olden times with the present technology; then to conclude that our predecessors were so due to lact of insufficient knowledge or advancement of science & technology. Implicitly, this leads to erosion of cultures - appreciating all cultures while living one's own is different from disregarding all cultures. Hence we see many variations due to this tendency; more often the question seems to be - "why not?".
Physiological reasons account for one.
Psychological reasons may also be the reason - the perception of the majority of women (by women themselves) is that they are secondary to men. This kind of equating with men has led to competing spirits. Starting from dress, manners and hairstyle, women are trying to show that they are equal to men by aping them. Moreover, literacy has increased among women (relatively) and they see themselves as self-sustaining entities. There may come a day when the word femininity and masculinity may see the end of usage even in grammar books. Such is the mentality now. This attitude of girls may urge them to look for an equal partner because an older partner, even by virtue of his age, implies seniority and some individual women may resent that.
Another psychological reason could be the inability to cope up with trends - Fashion, jargon, music, movies etc. Girls who move with males in their school and college might have appreciated the flair of certain friends. They would have relished such a character and would have attributed it to their age and exposure. A much elder person may have lost touch of enthusiasm or the trend and hence the reluctance of some girls to go beyond one year or max two for their life partner.
But age difference or not - true love for each other and for each others' families could potentially heal any sort of traditional deviations...
In my opinion, I would say a younger girl would enthuse the family with liveliness, care and affection while an elder boy would be physically and mentally strong to protect and care for her....
Elder boy & younger girl = maturity & enthusiasm
Boy & girl of equal age = maturity combined with ego
Elder girl & younger boy = dominance
While the above may be generally true (in my opinion), there are other variables like money, circumstances, peculiar individual traits etc which have their own way of deciding things.
Conclusion: It is better to go with tradition in certain areas.