we usually do during spring vacc and implant them, if you wait too long some will start getting bull necked. I have waited until early fall and done at that time also and been ok.
We do some at a couple of days old. The others I wait incase they are bull worthy and we show them as bull calves. Those ones are 6-9 months old when they are castrated. We bought a california bander last year and it is FANTASTIC!!!!!!!
We cut practically all of ours at around 3 months of age. Much older and they start looking "bully". I do know plenty of people wait as long as 5 months.