A buck can start spraying at two months or less old. Their musky smell comes from their scent glands and comes in mainly at the start of their first breeding season. Some bucks' smell will fade when they are out of rut, but they always smell strongly when 'in rut' during the breeding season. A buck will normally lose almost all of his smell when wethered, but even our wethers sometimes try to spray themselves so the older a buck is wethered, the more buckish he will probably remain. A buck kid wethered at six-eight months isn't too buckish to act like a wether, but will still be a bit more 'bucky' than a wether who was wethered at several days/weeks old. There are quite a few posts on here about wethering bucks, so if you do a search you will probably find a lot more info.
I kept two early 2010 buck kids intact through the 2010 rut season neither acted bucky that fall. This summer I wethered one of them. The other while a bit obnoxious (mounting behavior that I see often in even very young bucks) hasn't started to get real bucky, IOW hasn't started perfuming himself up for the girls. My two 2011 buck kids aren't either. I will likely wait until spring to wether them.
I have a 2009 angora buck and this (the 3rd rut he's been through) is the first rut he's acting bucky, peeing on himself and smelling very aromatic!
I wethered my 5 year old angora buck last fall just before the rut and he failed to get all smelly for the girls.
My limited experience is that neutering them later doesn't have much, if any, of a long term effect on how bucky they smell.