There are other considerations for breeding.
She should never be bred when she is growing up herself, she need the calcium and minerals for herself now, not egg production. She also needs to bulk up herself to support being gravid and all that it does drain on a female. Egg laying is very draining on a female and exhausting to her for a few weeks after, you want her fully mature so that she can bounce back from it.
It also is muscle developement, too young and they are more prone to egg binding, reabsorbtion and prolaspes that can affect them for life, prolapse can also be a problem for males bred to young as they need full muscle development.
Waiting til they are mature adult themselves is an easy price to pay to be assured you give her every advantage for a healthy long life and her offspring the same chances she was given. Her clutches will be larger, healthier, more likely to survive those first few months that so many fail to thrive in.
She will become a mature adult between 18months -2 years