It's more than age it's a combination of age, weight, proportion
For example I will breed a 1500 grams female that is 18 months if she has the right built (proportions), just like I will breed a 1200 grams female that is 4 years old.
Usually the bigger the better, smaller females are often not worth breeding as they will give you small clutches however if the female is mature (4 years and over) but still small (poor feeder etc) the clutch may be larger and it can have some advantage such as jump start her appetite.
Males while you can breed them small and young you can also over breed them and kill them if not careful, in my experience the best males are 650 grams and over.