how old before I can determine hens/roosters?

[birdginia]

I got two chicks from my daughter - her first grade class hatched and my grandsons wanted me to raise and add them to my flock of seven. My first fall/winter/spring has gone so well with my girls. Definitely cannot keep any roosters, and not sure about integrating with flock unless they are both girls.

Thanks!

 

[duckman14]

What kind are they? And how old?