From "Meet Sam: the Russian doll helping trans children with identity struggles", CTV, 2018-05-26:
"Research is very clear, that gender identity starts developing at around two to three years old," explained Annie Pullen Sansfacon, from Universite de Montreal's school of social work. "It will start consolidating at around eight, and may be expressed to others during their lifetime."
Professor Sansfacon said tools like the Sam doll are an effective way to educate children in these age groups. But the stakes are high, she explained, because statistics show that 41 per cent of transgender individuals will attempt suicide at some point in their lives -- unless they get early support.