Typically once the rack starts going down hill the deers sperm starts to degrade. There is nothing saying you won't end up with good deer out of him still however your chances are going down. Plus if he's been breeding from 4 1/2 to 8 1/2 it is likely that you will get some inbreeding between father and daughter. But that really depends on the deer herd in your area. If there are enough deer than very little will occur. Typically 1 buck can service about 30 does. My personal Opinion is take him out of the herd at about 6 1/2 or 5 1/2 he's been breeding since he was 2 1/2 years old most likely anyways. You have his genetics in the herd. What you are trying to avoid is if you see a 3 1/2 or 4 1/2 year old deer that does not have a good rack. If you happen upon one of those then you try to remove it depending on your management practices.