Thy only person I knew with GID said they knew from a very young age, but also always theorised that his mother may have been partially responsible- a combination of nature and nurture. As by far the most educated person on the subject I've met, I'm inclined to agree with him, though I do think that 5 is too young to make any medical diagnosis. That said, you face more and more pressure from society's expectations as you get older, and this person was eventually driven to attempt suicide before leaving the country to start a new life abroad.
Fortunately, the kind of people who became his close friends though life were very open minded and accepting individuals, and the only one who took any serious offence did so because they were upset that he didn't feel he could have told then years before.
I'm no expert on the topic, but I suspect I know a fair bit more about it than most gd armchair psychologists.