There is an olddd thread here (waaaaaaay back) about Matilda and Stephen (possibly in the "mysteries" board). At any rate, At the time which Henry I married Matilda to Geoffrey, Stephen didn't have a whole lot to offer. He was not a first son, don't forget. Whereas, Geoffrey was the heir to Anjou and would eventually bring that into England's budding "empire".
Perhaps, Henry I, was also aware of Stephen's "short comings", in the, ability to rule, department. And while, I personally, like to think there was a "romance" between Stephen and Matilda, it IS only speculation.
The "age difference" in those times and especially between royalty/nobility, was a moot issue. It would not even have come into the equation. The only time it would have been a consideration, would have been when heirs were needed and the woman was past "child bearing" age.
I wonder if the whole "romance" issue of these two was really purpotrated by Eleanor of Aquitaine. Her "court of Love" (especially unrequited love) was noted for these type of stories. (just a thought)