4weeks minimum age, but if you plan to neuter them later then 6weeks, remove the buckling(s) to a separate pen from their sisters/dams. They can breed as early as 7weeks, so you have to be very careful.
A good set-up is to have large dog chain-link dog kennel to keep all the bucks in until you get them castrated.
Most people band miniature goats, but when they're big enough, you can use other tools.
It is possible to castrate older goats; over 4months, without much trouble at all if you use a correct tool.