i dont know if it applies specifically to rabbits, but there is some decline in fertility in most mammals with age. there is also an increase in genetic and structural abnormalities in the sperm. mostly this will just lead to subfertility/infertility but could lead to the production of abnormal offspring, depending on the severity fo the abnormality (i.e. significant but not enough to cause failure to implant etc so offspring still develop). So there would be some potential to produce disabled offspring. but i dont know at what age that would occur with rabbits, if at all. I think generally if the buck is a bit of an "old man" in himself he probably needs retiring!