* Not all trans people remember ‘knowing’ at a young age, and such early-life knowledge is not a prerequisite for being trans; early childhood memory is a spotty thing, and different people develop differently.

That said, for those that do fit the ‘early onset’ profile, the age of what one might call ‘trans awareness’ is often around 4 years old +/-. This fits with early childhood development studies which show first awareness of gendering at around age two, development of gender stereotypes at age three, and more sophisticated self-typing by age four. Thus, if a gender incongruity exists, this is about when one would expect it to show up. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3747736/