I have a daughter who didn't really have many "girly" interests until preschool - I'm pretty sure most kids start to learn gender as a social construct from their teachers and peers.

Honestly, at this age, I just frame things like "many girls like to have long hair, but boys can have long hair, too" when I'm asked questions about gender and expression. The emphasis is usually on the fact that it's ok for people to do/like things that are usually associated with the other gender, and encourage self expression.