I'm trans, at what age should the niece abd nephews find out?

I'm a trans woman and have been on hormones long enough that it's had its intended affects. Their mom knows and told the oldest boy (14) a year ago (then 13) because he insisted on going to the lake.... He took it in good stride, his two biggest questions were "why does anyone care?" and "so I call you my Aunt now... 

The younger brother (12) had to find out a couple months ago because they were spending the summer on the farm to learn about chores and get away from covid... His only complaint was we didn't tell him a year ago and we went swimming without him... Also, at 5'4 he swears he will beat up anyone who has a problem with it... 

So since they now come out every other weekend their little sister (10) wants to too.  Of course, it's a farm, so I retained a lot of my male clothes because my heels dont look good with cow pies on them, so aside from binding my chest I wouldn't need to do anything to my appearance but there are so many other things that could give me away...

So it's gonna have to happen eventually... At what age should she know that im transgender?

(Btw I'm okay with never having to have that convo(