Most chickens that are farmed commercially and slaughtered and sent to processing plants etc are raised to gain weight really fast and those chickens are usually slaughtered at between 5 to 7 weeks of age.
However at home when you raise chickens the best age to kill a chicken or slaughter a chicken for it's meat is before 8 months of age.
Preferably you should be able to slaughter or kill your chicken for meat after around 3 to 6 months of age.
After 8 months of age the chicken can still be slaughtered for meat but the chicken meat from a chicken over 8 months of age is tougher and may not be as good.
So if you want the best meat from your chickens slaughter them before 8 months of age.