It's not that uncommon to come across a book, movie, or argument online where a "May-December marriage" (i.e., significant age difference between the partners) takes place. However, in the United States and Europe were they really as common as people thing, or did most people tend to marry fairly close to their own age? Or was this something that was a result of the mortality rate?
There's always more to be said, and that's particularly true when it comes to questions about social trends across wide periods of time, but some time ago I wrote an answer to Why did large age gaps in marriage go from being common to being unpopular? that might be helpful to you.