I was still on line when your message came through so thought I'd reply straight away. Re early castration in horses, it was fashionable for awhile to geld as young as 4 months, but then some veterinary authorities began to suspect a link between this early castration and musculoskeletal problems later in life. Some racehorses are in serious work and galloping by 20 months of age and they're nowhere near mature (no wonder so many 2 year olds break down), so it's only common sense to avoid anything that's likely to add to this problem, like early castration. I don't know if any actual controlled study was done on this, I think it was just a matter of observation and putting two-and-two together.
Re spraying cats, I've never had purebreds but have had the odd cat not castrated before 9 months or a year old or so, usually because they've turned up on my doorstep at that age. Thinking back, I'd say all of these late-castrated cats were sprayers, some worse than others. They were worse if someone strange had been in the house (and of course if a strange cat had been around outside), and often targetted things like my husband's shoes - it sure was a good way to get him to be tidier! I don't remember any of the cats castrated at a younger age being sprayers, at least not obviously.