Can you get a dog spayed or neutered at five years old?

Yes, but it will not be as effective. The dog will not be able to reproduce, but if the dog is a boy, he will not stop spraying things, nor will he stop humping your legs. I can't think of a reason it wouldn't be as effective on girls. I highly encourage you to get your dog/dogs spayed/neutered!

Humping and such is actually more behavioral. It is a dominance issue more than a hormonal. It can be broken with or without neutering. However, having a dog spayed or neutered can prevent other common problems. These include unwanted pregnancies and cancers of the reproductive system.