We routinely keep bulls for 6-10 years... have several smaller pastures we summer cattle at so need several bulls. If the bull passes his BSE and has put decent calves on the ground before, he gets to stay.... We have enough different places to pretty much not use a bull on any daughters. If his attitude gets bad, he goes... If his attitude is good, they will often still be here at 10-12 years old... Then it is a matter of if he covers the cows, and his feet/legs are good.
Bought a mature bull in 2015, farm dispersal, don't know his actual age... keeps weight on pretty easily.... Passed BSE this spring and we really like his calves... he will stay until we have a reason to ship him. He never challenges fences, does not fight with others in the bull lot, comes right to you when you call and will load in the trailer out in the pasture...
We don't chase the newest and latest/greatest.... we want them to sire decent calves, 70-90 lbs mostly, quiet and easy to work with, gets them bred on time, decent confirmation and average and better calves. If it is an older bull, so be it...
When we buy a bull it is for him to be around for 5-10 years... Just sold an easy calving bull that was bought in 2012, and another that was bought in 2013...same breeder; small 60 lb calves, quiet, not rough with the heifers and never hurt any breeding... Weighed in the 18- 2000 lb range when sold...They did exactly what we wanted and I hate that one got feet problems and the other got hurt. But they were "old" .... and at that age they were entitled to show some problems.
Longevity is VERY important to me.... and good type, good legs and feet is part of what gives them longevity.