We (well, not US, but you know what I mean) gelded a 7 year old QH stud, and he still acts like a stud until this day, and he's 12 now. I think it's just habit for him to act that way. He still squeals at mares, and just acts studdy. Also, there are some "geldings" out there that are not actually true geldings, they are proud cut, which means they still think they're a stud, but they can't breed a mare (or get her bred, I should say, they'll still try). Whoever cut these of horses didn't do it correctly, and have basically left parts still in there that needed to come out.
I've heard that if you do castrate an older stallion, if you work him hard and exercise him daily for a month or two, the hormones eventually will get burned up out of their system...but who knows?