Young abbots

Marco Riebler (Salzburger Nachrichten)

Media reports have drawn attention to the fact that the new archabbot of St. Peter's in Salzburg, Jacob Auer, is only 33 years old. The first thing to recall is, of course, the biblical dimension of that number and its significance in the life and mission of Jesus Christ. And then there was Bernard of Clairvaux, who was made abbot of Clairvaux in his mid-twenties.
Spiritual and hagiographical meditations aside, it turns out that 33 is not even that young in terms of St. Peter's very own history. St. Peter's archivist Gerald Hirtner quickly calculated that Jakob Auer ranks fourth (!) in terms of young abbots. In other words, he isn't even that young. There were significant and hugely successful abbots elected to lead St. Peter's at a younger age, namely Amand Pachler and the supremely significant Placidus Mayrhauser.
These are only some points of comparison from the Early Modern period, for which precise biographical information is available. With an abbey as significant as St. Peter's, whose history reaches back into the seventh century, there were certainly dozens of medieval abbots elected while in their early thirties. 
Postscript, 5 March 2025: Blogger Georg Schrott picked up the ball on this topic and played it back regarding abbeys in the Upper Palatinate in Bavaria. See this report on young abbots there.