I only see that term as relative to someone's management goals, if they have one... mature buck is typically determined by the age of 4.5 yrs or older I say. As far as abuck being a cull buck...well I guess if you manage for BIG mature bucks, then it would be a deer that is 4.5 yrs old with no potential to grow any larger or has poor genes so to speak...a 4.5 yr old with a score under 125" maybe a cull buck to some people and places, again it is relative to location and goals. just my 2 cents, but I have no experience in aging bucks or managing them....