Why are there so many age disparate relationships?
One of the attractants in the lifestyle is experience and confidence. Those two traits "usually" come with age. So, many younger lifestylers are drawn to older lifestylers who know there way around a dungeon... or who have the patience that comes with experience to bring along a younger partner... and certainly we have the "advantage" of a "slowing sexual response", which many young subs find attractive.
On the other hand I was exceptionally experienced and confident in my 20's and I imagine there are a number of younger dom/mes out there who meet that criteria today. It may take a while longer to find someone who matches your personal criteria exactly and for some, the search and discovery is the best part of the quest.
A second driver for age disparity is that many people enjoy age play, look for older partners who will be Mommy or Daddy to their Little Boy or Little Girl, Prince or Princess. For many, an actual age difference augments the fantasy. For others, size helps (a small or petite "little" and a big or tall "elder".)
And finally, all that said, if you have the opportunity to attend a play party, try out an older dominant. See what s/he has to offer. You might be surprised.