I chose fully matured. While little bitty chicks are super cute and I can't help but fawn over them, I so appreciate when I don't have to worry about getting chick feed, checking the heat pad, supervising integration, worrying if they're really girls or not, etc.
The teenage phase is my least favorite. Chicks that were tolerant to you suddenly go fleeing if you even look at them, and they're awkward with the adults (just had one of my pullets stupidly play-challenge a hen, who nailed her several times in the back and then grabbed a beakful of feathers before letting her scurry away).