Well, the younger the chick the greater the chance of incorrectly sexing it and prying open a pullet to look for testicles that are not there. I guess you could operate on very young chicks that can be feather- or color-sexed, like White Leghorns and Sex-linked breeds, but their testicles would be so tiny they would be more difficult to locate & remove. I don't know just how young a person could successfully caponize a chick. The ones I've done were between 9-12 weeks. Their testicles were the size of dry navy beans.
I think the purpose of caponizing is to make them grow meatier, just like other castrated meat animals like pigs, cattle, sheep & goats. I really don't think that the male hormones affect the taste or texture of the meat, but instead just regulate & limit the animal's growth. When their bodies are de-programmed for reproduction, they redirect their energy into growth.
I also don't know how old is too old for caponizing to have an affect. I guess like with other animals, the later you neuter them the more time they've had for their hormones to have an affect on their growth & behavior. I wouldn't want to try it on a fully-grown rooster, or even one much past 15 weeks. I'd try to operate on them between 7-14 weeks, as soon as I was fully certain of their sex, not after they started to act cocky.
I've processed & eaten intact roosters, at 20-24 weeks their testicles are the size of large peach pits. Their meat has been tasty & tender, especially when cooked slowly with moist heat, but there just isn't as much of it. The largest I've had was barely 5 pounds, most average around 4. I would like these standard and mixed-breed cockerels to grow bigger by butchering time.