Ten years old, that's when it's decided, at midnight the minute you turned ten, a name appears on your right wrist.
For Castiel and Dean neither knew wether they would meet their soulmate.
Castiel hid his and Dean was ashamed of his.
Dean has a secret, he can't get close to anybody in fear of them dying, because the monsters are always there, and they hurt everybody he loves.
Castiel doesn't know monsters exist, he refuses to believe they exist, demons, vampires and werewolves, they're all stories, until he's proven wrong, he will alway believe that.