Could it be possibly be something like it got darker when he was five? Or everyone considers it as a birthmark but it's a wound from some accident? Burn mark? But if it's a birthmark that means he has had it since birth. Dunno why they say "since you where 5 years old"
But how did he get his birthmark?
http://www.mangago.zone/read-manga/bj_alex/uu/bj_alex-chapter-2/3/
http://www.mangago.zone/read-manga/bj_alex/uu/bj_alex-chapter-2/4/
http://www.mangago.zone/read-manga/bj_alex/uu/bj_alex-chapter-2/8/