I just might they are fine to breed at this age. Best to not put a box in now as bondage breeding isn't ideal best to wait if they bond. However they may never bond give them time. What set up have you got? A pic? How long have they been together
If they are the birds from your other post please don't breed them. Tiels (both male and female) need to be at least a year and a half before breeding. They need to be bonded before you add a nest box otherwise it will not end well for any babies they may have. If they do not bond to each other then they are not meant to breed. Siblings should not be bred to each other either.