We use all kinds. We've bought them as yearlings, two year olds, and we've bought three year olds too. Three times we've bought three year olds who were previously herd bulls in purebred herds, and they've all been really really good. Way above and beyond average.
Last year we turned out 8 bulls. Two yearlings, two 3 year olds, two 4 year olds, a 6 year old, and an 8 year old. They got the job done. We're not afraid of an 8 year old bull, especially if it's a Limo. They go on and on and on, like Energizer bunnies. 8) 8)
Disadvantage of a yearling..... Sometimes yearlings can fool you, and by the time they've finished growing they don't really look like you thought they would.
Advantage of a yearling ... You can raise him the way you like, and make sure he doesn't get 'burnt out' on high energy feed.
Disadvantage of a two year old .... Was he passed over as a yearling? Maybe he was a runty calf, and needed a bunch of extra feed to catch up.
Advantage of a two year old ... You know what they are going to look like, and they are just that extra bit more durable.
The important thing is to get a bull that works with your cows, and then deal with his age. It's the genes you're after anyway.