Depends what you mean by gender. I'm using the definition of gender as a social construct, which obviously can't form before a child begins to observe society. If you're defining gender as a hormonal response, then sure, it probably develops in the womb. But I think that's not very meaningful when we're talking about how to socially identify a child. I think kids should only get gendered pronouns once they understand gender in society.

And of course, they should always be allowed to choose their gender. The reason I suggest a tradition of "gender choice day" is because most people are wangrods and will gender their kids too early, if they aren't given a tradition with clear parameters. The idea of "gender choice day" is very similar to cultural milestones like Christmas or marriage, where we have traditions of what not to do. Don't peek at your presents and don't let your husband see you. People respect those boundaries because there's a clear moment when the boundary is lifted, and it's framed as "spoiling" the special moment. I want to harness that stuff for something that actually matters. No gendering kids before gender choice day!