Ok, I see where you're coming from. Yes, the chicken has grown its adult feathers and size, while the lamb still has some growing to do. But in terms of the portion of its feasible life span, and in terms of being "mature" enough to eat, where the quality of the meat would drop off afterwards, the average commercial meat chicken reaches meat maturity quicker and misses out on more of its life than your average lamb does. The commercial meat chicken generally has a worse life than a lamb does. Yet people feel more sympathy towards cute little lambs than chickens.

Even if the lifespans and conditions were exactly the same, the point I'm making is that people feel more sympathy towards one type of animal when another should be at least just as worthy of it. The fact that it's a "baby" is irrelevant, because the justification for feeling that way about babies is that it hasn't had the chance to live a full life.

Saying it's ok to kill a chicken but not ok to kill a lamb is like saying that it's ok to send an underage man off to war because he hit puberty early.