Here is a Wikipedia article on
Kim Petras who claims to be the youngest known transsexual person to have had a sex change at 16:
http://en.wikipedia.org/wiki/Kim_Petras
Google Image Search about Kim
Note that sex change surgeries are done to newborn inter-sexed infants, but they are not counted as transsexual.
There are also numerous cases where "normal" male infants that had suffered genital injuries or were conjoined with a male twin sharing only a single set of genitally, and they were forced to under sex changes into girls. But in adults years, in nearly every case, they grew up to became transsexuals (identifying themselves as male), and many of them ended up getting FTM hormone therapy and surgery.
Because of the bad results in the past, these days most medical institutions in western countries no longer reassign boys into girls solely because of male genitally injuries or genital amputations.
These cases, along with Kim's and other transsexual cases should indicate that gender identity is entirely nature and not nurture. Perhaps in the future, babies or even fetuses can be screened for transsexuality and then undergo sex changes after birth or be aborted during the earlier stages of pregnancy.