Associations between childhood sexual interactions with other children, and preferred and actual age of sexual partners, as well as adults' sexual interest in children, were explored in a sample of 1312 Finnish male twins. Experience of sexual interaction with other children was associated with lower minimum age of preferred and actual sexual partners in adulthood. In addition, such interactions were connected to an increased likelihood of adults' sexual interest in children under the age of 16 years. None of the participants who reported no such interactions had sexual interest in children in adulthood. In addition, the presence of a female co-twin was associated with higher levels of childhood sexual interactions and lower minimum age of preferred and actual sexual partners. Finally, the extent of childhood sexual interactions was not affected by genetic factors, suggesting that the identified association represents true environmental causation. Experiences of childhood physical and sexual abuse were positively related to the extent of the childhood sexual interactions with other children. The results support the role of conditioning in the development of sexual age preferences.