i laughed when i saw your question. not at you but at goats. bucks will try to breed as long as they have breath in their body. now how long or at age will their sperm no longer be viable is a good question and truly depends on the buck. i have had breeding bucks until they were 10-12 years of age and others that were shooting blanks at the age of 4-5. just depends on their overall health and genetics. what kind of buck do you have or are thinking about getting?