I don't know the details of how this has been concluded. But when dealing with sexual identity what they are mainly talking about is whether a child identifies as a boy or a girl and all of the associated social constructs that come with that. Whether sexual attraction is part of that or not depends on who you talk to.

According to Freud's psychosexual development model, the phallic stage, occurring between the ages of 3 and 6 (and therefore coinciding with the aforementioned hardwiring of sexual identity) is the stage during which a person's sexual orientation will form.

I personally don't believe Freud's psychosexual development model is accurate. But it seems fairly clear based on everything I know that sexual orientation is hardwired very early in life, if not at birth. Therefore, I think placing the maximum age at which it could still change at 6, so that it coincides with the development of sexual identity and; for those who are Freudian; the end of the phallic period, is a safe bet.

I personally believe, based on my experiences, the experiences of people I know, and the accumulation of all the knowledge on the topic I have been able to glean from many sources over the years, that chances are that sexual orientation is hardwired by the time a child reaches kindergarten, in the very first few years of life. Stretching it to the age of 6 is the result of limiting myself to only solid scientific evidence on the topic. But I have little doubt that if the studies were ever done, they would find that sexual identity is hardwired long before that.