As far back as I can remember, which is back to age four, I had an erotic interest in girls' bare feet. (Even though I had no idea what "erotic" meant at that time.)
By age 7 or 8, I knew that I liked to be tied up and tickled. (Again, not realizing that it was an erotic enjoyment.)
By the time that I was 13, I had read Richard von Krafft-Ebing's book (in English translation) "Psychopathia Sexualis," and I understood that I was not alone in having non-standard erotic interests.