He regularly used drugs such as marijuana, LSD, cocaine and methamphetamines.
Cowan was born on 18 September 1969 in Bunbury, Western Australia, to Marlene, a homemaker and RSL worker, and Peter Cowan, an Australian Army Major and Vietnam veteran. He was the third of four boys. The Cowans led a strict household and gave their children a Catholic upbringing, and were, by many accounts, a "good family". Further, Cowan's father was often absent due to his Army responsibilities, and suffered from PTSD. In the early 1970s, the family relocated to Brisbane, Queensland, eventually settling in the suburb of Everton Park.
Cowan and his three brothers attended Marcellin College, now known as Mt Maria College, an independent Catholic high school in Mitchelton in northside Brisbane. Cowan's year nine English teacher claimed Cowan was somewhat of a "bully" who taunted other students, would proudly parade around his erections and was "despised by his peers". Conversely, other students at his school reflected on him as being an "ordinary guy" with few hobbies or social interests who "seemed alright".
Cowan was also bullied in school and dropped out in year ten and began working odd jobs throughout his late teens, never holding down employment in one position for longer than a few months. Cowan also struggled to come to terms with his bisexuality. He also proved difficult for his mother: he was a "handful" as a child and progressed into a difficult teenager, occasionally stealing from her. His problems escalated as he became involved in petty crime and his first criminal conviction came at the age of 17. By that time he regularly used LSD, cocaine and methamphetamines. He later stated of his drug use: "They do fuck your inhibitions up … you know, your boundaries."