nose rings are mandatory at most of the shows i am familiar with. Age does vary, but most shows I am familiar with say that a nose ring is mandatory on bulls 12 months of age or older. Most shows don't seem to police it providing a humbug is used at all times, except when a bull is tied in his stall. Personally, I think all bulls over 12 months of age should have a ring. A few years ago I had a super quiet, puppy dog of a bull turn on me, and ended up putting me through the wall of a shed. This bull had been shown all over North America (Louisville, Denver, Edmonton, Toronto etc) and could be haltered anywhere. I am certain if he had not had a nose ring, I would be dead today, as he really snapped. I was only able to regain some control after I got me fingers in his ring. I still have a hole in my leg, 20 years later to remind me of this event. This bull was shipped to market within 1 hour of this event. There are enough quiet cattle on the planet that I can own so that I don't have to keep an animal I cannot trust.
It was a lesson I will always remember and I always treat bulls with respect, no matter how quiet they are... or seem to be.