People tend to choose romantic partners who resemble their opposite-sex parents. What remains unclear is when in ontogeny are these mate preferences formed. We have investigated the timing of possible sensitive period(s) of learning paternal characteristics by comparing similarities between fathers and romantic partners in heterosexual women who started cohabiting with nonbiological fathers at different ages between birth and 15 years (N = 250) and women who cohabited with their biological fathers up to 15 years of age (N = 1,613). All women indicated mate choice-relevant demographic, physical, and personality characteristics of their biological or nonbiological father and partner. The probability of a sensitive period was estimated using year-long windows. Sensitive periods for learning paternal characteristics were detected in 16 out of 21 characteristics. Characteristics clustered into those with sensitive period in early childhood (age 1–5 years), and even more so, during a sensitive period in adolescence (9–15 years). For demographic characteristics, the sensitive period was 12–15 years of age, for physical characteristics 9–13 years of age, and for personality characteristics, we found no clear age pattern. Although the timing of a sensitive period is not the same for all characteristics, our results for the first time suggest that women learn paternal characteristics which are later used in mate choice during two time windows and mostly during adolescence, thus later t