Warning: Spoilers ahead for Let Them Live!: Unpublished Tales from the DC Vault #3!
Batman's training secretly started long before fans even realize. Let Them Live!: Unpublished Tales from the DC Vault #3 reveals that Batman didn't only begin training in his young adult years - Bruce Wayne began acquiring skills he would use as the Dark Knight when he was still very young.
Let Them Live!: Unpublished Tales from the DC Vault - written by Elliott Kalan with art by Mike Norton - is a digital-first comic on DC Universe Infinite. So far the series has released issues featuring the Suicide Squad and Nightwing. The third issue focuses on Batman and a new case that reveals more than just criminal activity. Batman is known for being a top-notch detective who knows many forms of martial arts. He does not have any superhuman abilities, but he makes up for these by using his mind, physical skills, and other skills he has acquired during his life.
"Flesh Made Word" is written by Scott Bryan Wilson with art by John Paul Leon. It pits Batman against a new villain whose operations are centered in a library in Gotham: the Willoughby Z. Tchalgadjieff VII Antiquarian Library. Batman has found a book that leads him to the library and may connect to people who have been recently disappearing. On top of being a usual Batman detective story, this issue also shows how - and where - young Bruce spent his time.
While investigating in the library, Batman mentions having spent thousands of hours there as a child. He knows the variety of the collection and is familiar with some very specific works. One is An Analysis of Seed Dispersal by Bats in the Geomorphological Subsystems of East-Central Europe. The other is Functional Anatomy of Maxillofacial Structures. His fascination with bats and highly specific information that would seem very boring to most people began earlier than Bruce's physical conditioning. Though it is well-known that Batman has a superior intellect, most comics focus on his vigilante activities. Training himself to be the world's greatest detective was not an overnight feat and his scientific skills came from many years of study, long before many fans may suspect.
In this issue, Bruce mentions an instance when he broke into a librarian's office, specifically to see texts involving criminals that the librarian claimed to possess. These were supposed to include prison letters and manifestos - although Bruce never did find them. His desire to seek out evil and try to understand it in some regard is truly a lifelong obsession. Though this may seem relatively obvious considering the circumstances of his parents' death, rarely have fans seen when Bruce began developing his detective skills as a youngster. It can be assumed that as a rich child from a well-connected family, he received an elite education, but it also seems as though Bruce was an independent learner. He is the world's greatest detective, but he didn't magically learn everything. Batman has put in many years absorbing information, reading in the library, and has practiced vigilante activities, all with the motivation of facing evil head-on.