Complex sentence: As a young adult in the early 1620s, Hall decided to adopt a man's hairstyle and "changed into the fashion of a man" in order to follow Hall's brother into the all-male military service.
Split into simpler sentences: As a young adult in the early 1620s, Hall decided to change their self-representation from female to male. Hall adopted a man's hairstyle and "changed into the fashion of a man" in order to follow their brother into the all-male military service.