The psyker mutation usually manifests early in life, but there have always been exceptions. So-called ‘nascent’ psykers may be born with the mutation, which lies dormant, or may mutate after exposure to the corrupting influence of the Warp. In this current age, the baleful reach of the Cicatrix Maledictum awakens psykers in numbers never before seen. "It sometimes seems as if anyone is a potential psyker".
In game terms, the Psyker Talent is available for purchase just as any other. However, to become a psyker represents a profound change for a character. If you are considering the Talent, think more carefully about it than you would for other advancements. To become a psyker is to pass through a gate from which there is no return.
When you take the Psyker Talent, you are, by default, unsanctioned. All psykers are unsanctioned until they undergo the ancient trials and trainings of the Adeptus Astra Telepathica. Whether you surrender yourself to the Black Ships or continue your work without sanctioning and risk the wrath of the Ordo Hereticus is a matter to decide with your GM and fellow players. The choice is sure to have a major impact on the campaign, and will not be compatible with many Patrons — some of whom count hunting unsanctioned psykers among their most important duties.