Yes the age does matter, I remember hearing one of the sexers talk about it when I was asking him questions. I guess the older they get the harder it becomes and the more likely to hurt them. Don't hold me to it though. I think it's more of an accuracy thing when they are younger.
Vent sexing is somewhat traumatizing to chicks and may even cause death. At hatcheries chicks, and poults, are re-examined for quality a few hours after vent sexing by trained professionals.
After vent sexing the are packaged and shipped, they aren't re-examined.... Some do die but I've only...