how and when do we know our gender?

Can someone as young as (insert age here) really be sure? 

I am always struck by this question when parents of trans youth ask. I often respond by saying, well how and when did you know your gender? The most frequent answer is- well I just kind of did...I always have. 

Child development theorists say gender identity development occurs by age 6. This has supposedly been a widely accepted notion for years. If this is the case, then why do we expect cisgender kids to have an understanding of their gender identity by age 6, but send messages to our trans and nonbinary youth that they are too young to "be sure" or truly know themselves in this way? Trans youth are too often asked to put living as their authentic selves on hold until they can articulate their gender identity with a greater degree of certainty. Society seems to accept that cisgender people simply know their gender intuitively, while trans folks are required to somehow prove who they know themselves to be to the rest of the world.