Two issues with gelding older stallions: one, complications, two, disappointed expectations for changing behavior.
Not to be a wet blanket, these are just the facts: gelding older male horses is not always 'effective' in making a behavior change. The changes may be partial or there may be no changes at all.
Gelding older stallions tends to involve complications many more times than with younger horses. The complications are more severe and more frequent.
Part of that may be, that stallions that have been left entire for many years, tend to develop tumors and cancer in this organs, which may complicate the surgery. When one gelds an older animal one also may be dealing with retained testes or 'stallion hernias'. Stallion hernias seem to occur more often among older stallions.
In some cases, a stallion MUST be gelded - such as in many cases of hernia, cancers, etc. That is more to save the animal's life than to cause any behavior change.
And it doesn't always change behavior, much of which, by the time the animal is 'aged' (over 8) or teenage years, is learned and habitual. Keep in mind that in many teenage stallions they are already 'slowing down' in the hormone department - if they are still acting like Mr. Stud a lot of that can be habitual.
Gelding at any age, even quite young ones, doesn't always remove all reproductive behavior from the male horse, and this is just even more so with the older animal who has established habits over years or decades.
That is why any horse that is gelded when it is older is referred to as a 'stag' - because they quite frequently remain stallion-like. Nearly all will retain the 'secondary characteristics' - the large crest on the neck, etc.
Additionally, if the stallion has already been used for live cover, gelding is less likely to erase his fond memories of that process.
The owner should discuss with her vet, the 'pro's' and 'con's' of late age gelding, and why they want to do it and what they expect to achieve....as well as how mad they're going to be with the vet if nothing changes!
Even though people have seen these 'stags' change in behavior it just does not always happen, or happen to the degree the owner would like. I don't even know if it would be 50-50.
I have a 'stag' - a horse that was gelded at 4 - 'stag' is almost not even appropriate at that young age. This horse's behavior did not change at ALL for one year. He had to be handled as a stallion for a YEAR after the gelding.
Plus, he NEVER really lost his stallion behavior entirely. I would say, for oh, 5 years after the surgery, he was still about 80% stallion. No, no error was made and no tissue was missed, and 'both were down'. Today, even at 20, he is still rather 'different' from other geldings and can bring and keep my mare in heat.
In general, I would say that 60% of the time, there is a behavior change of SOME amount, but it isn't always everything the owner anticipated. Given the risks with older animals(complications and insufficient changes afterwards), it's not always a simple decision.