To make things clear, I want to state that I have no doubt about whether people are transgender and what that means. I simply am asking about what age children are able to comprehend their personal situation.
The family of an adult who recently came out as transgender sent me the PDF by PLAG titled Our Trans Loved Ones: Questions and Answers for Parents, Families, and Friends of People Who Are Transgender and Gender Expansive.
As the parent of two young toddlers, I found it interesting that it states that children with gender dysphoria are aware of it as young as age two. See page 26-7:
"These children.... can start telling us as early as the second year of life, maybe even with their first toddler sentence..."
I related this to my husband, who expressed doubt that a two-year old would be able to understand that and felt it would happen more around age six. The rest of the PDF doesn't help with specifics much, as pages 17-8 just state,
"[O]ur sense of self (including gender) becomes apparent at a very young age; for most this is between the ages of two and four years old, and this awareness remains stable over time."
The document just treats this as established fact.
So where are the studies about this? How do we know that children understand not just that other people can be a boy or a girl, but that they themselves are one and that it will stay "stable over time"? Or, even better, are there studies that show that these children who tell their parents "with their first toddler sentence, 'Me not boy. Me girl.'" overwhelmingly identify as transgender (or gender expansive) at any point after they can hold an intelligent conversation? I mean, for months my 3.5-year-old has been correcting me and telling me that he's "not a little boy" but "a truck." I don't think he's going to identify as nonhuman when he grows up (but I could be wrong, and I would love him just the same). We want data, not anecdotes, even though I know hard data are tough to come by with young children.
Some related research was linked to from an article here: When do children develop their gender identity? but that article is more interested in studies that show children have a more fluid concept of gender and it only becomes rigid later on, which is in some ways the opposite of what the PFLAG document claims.