The age you disbud is determined by the rate of horn growth. Some you have to burn almost as soon as they are born, others you can wait up to 2 weeks.
When you feel little nubs they are ready to disbud. You don't want to wait till they are visable. At that point the horn bud has attached itself to the head and it is harder to stop the growth. Bucklings tend to have a wider horn base then doelings so doing them sooner is best.
I have a Wal lenk 125 watt disbud iron.
A disbudding box is so helpful for keeping kids still during the procedure.
(picture coming)
Here are some pictures of how I disbud.
First you shave the area so you can see how wide the base is and so you don't have that nasty smell of burning hair.

Next you place the disbudding tool over the buds for aprox 10-15 seconds (less if using a hotter iron). I also like to "tap" the buds themselves as you can see in the picture below.

it is important to get that copper ring. (the buds in this picture look glossy because of the aloe I put on it)
Note in this picture the piece of wood. You want to test the heat of the iron on the wood before you start. Once it makes a COMPLETE burn ring it is hot enough. Between buds wait for it to heat back up. Test on the piece of wood if you are unsure.

I then comfort the kid for a bit and either give a bottle (of a bottle kid) or return him back to his mother butt first. Why the butt? so that she doesn't reject him because of the burn smell. Doesnt' happen often but it can. She may not totaly reject the kid but for a couple hours refuse to nurse the kid. Best to avoid this if possible.
If the kid seems in distress (doesn't happen often) then you can give 1 tablet, 81mg, of asprin (not tylonal!!!)