I've talked about why we can't really talk about a single "average age" for marriage in later medieval Europe in this thread. For upper class people in Italy, yes, you find a stark disparity between 12-13 year old girls marrying 30+ year old men. The further north you go, and the lower down the social ladder, marriage ages converge. That answer goes into detail on the specific situations--it's not a blanket answer like "patriarchy," although medieval Europe was certainly a heavily sexist society.

Is there a particular region you are interested in, such as Europe? Because as stated this is awfully broad, but looking at how age difference in marriage has changed overtime in a specific place would be a good deal easier to tackle.