I recently found out that a distant great+ grandfather in the 1800s was 13 when he had his children with his 33 year old wife. Was this a common occurrence (a under aged male [or female] to have children with an older, adult wife [or husband])? Or even at any point in history?
It really was not - it is so uncommon that it's very unlikely. You may be interested in my previous answer on Why did large age gaps in marriage go from being common to being unpopular? - it only discusses large age gaps where the younger partner was the wife, because that was certainly the norm in these situations.
Can you explain why you believe your umpty-great-grandfather was only 13? Is there a possibility of a misidentification with a son with the same name?