A combination of the three - age because they have to have the maturity otherwise you are breeding for nothing - weight because they have to have the fat reserves for a healthy "pregnancy" and you have to look at length too because a 1500 gram 3 footer looks a lot different than a 1500 gram 5 footer - which goes back to having the proper fat reserves for the breeding season.


PS: I used 1500 grams as an example not a rule.