Breeding is a combination of age, weight and overall look
Sometimes breeding a 18 months female is right sometimes it's not (I have no issue breeding a 18 months old female over 1500 grams)
Sometimes breeding a 1100 grams female is right sometimes it's not (I have no issue breeding a 1100 grams female over 3 years)
Sometimes breeding a 2000 grams female is right sometimes it's not (I would not breed a 2000 grams female if she does not look right)
Etc