The youngest is the same age as him (well...two days younger); 5 months and she should start laying at any time. Any time he goes after her, he gets lambasted by the older pullets who have been laying since this summer and the 4 year old hens if he gets in their way.
Because going after the youngest isn't quite so easy for him, he has now begun targeting one of the older pullets (8 months) who just happens to be the smallest in the flock. She's a Hamburg and won't be getting any larger. The rest of the girls are roughly the same size and all with a similar build (though the youngest is turning out to be my largest girl - her mother was a Marans).
I was thinking that maybe because she is built differently, he's trying to run the Hamburg right out of the flock. Luckily for her, my two oldest hens (and EE and an SLW) seem to welcome her company and if she's spending any time with her flock mates, it's them. Otherwise she's off by herself or right at my heels. The rest of this year's hatchlings just seem too spastic for her liking to begin with and now he's chasing her away.