Sub-adult males I don't have a problem with, but females have to go through egg formation & laying and may not necessarily be internally developed enough to cope.
A 12 year old female human can have a baby (it's pretty common in my area...) but there are health risks and potential complications that would be greatly reduced in a fully adult female.
Breeding a sub-adult female Corn would be playing the odds amd if you choose to take that risk, it's yours to take. Just make sure you have money put aside for possible vet bills. Is it really urgent that you breed her? Would an extra year make such a difference? It might not be life and death to you, but it could be for her.
Good luck with whatever you decide.