Yes, you can geld a stallion at 8 years of age. You can geld them at any age. The problem with gelding an older stallion, is that their testicles are far more devolped, and need a larger incision to remove them. With a larger incision, infection is a highr risk.
Gelding a miniature is done through the same process as a bigger sized horse.
Animals, what animals? My children just have alot of hair!