Kent is right. Another reason for using older proven sires is that some people just don't like the risk of using semen off a 40% accuracy or less bull. If you just use the ~2 year old bull that won Denver in the current year, you are going to get some great ones and some years you are going to get cattle with substantial flaws. For some people those great ones makes up for the not so great years. Other people want a more consistent product with less mistakes out in their pasture. I encourage people to be out there on the cutting edge proving bulls. Somebody has to do it afterall, but we should all be aware that there are risks.