There is tons of info on this on the board, just do a search.
IMO, it is better to castrate them sooner, rather than later. Less stress on them when they are done real young, than when you do them at an older age. Also, less chance of infection I think if you decide to knife cut them (as long as you use sterile equipment).
We band them usually before they are a day old. Doesn't seem to set them back at all. If you are really worried about growth you can implant them at branding time, that takes the place of any testosterone you are losing out on by cutting them, and costs about $1/head to get a 15+ lb gain.