Neutering (unsexing) cats at four to six months is a good time. Some will go as early as eight weeks, but some vets think this might cause urological disorders in male cats later on. Siamese and related breeds mature early, so six months may be almost too late for them.
(...) mentioned that pets that reach sexual maturity will be more likely to have cancers of the prostate (males) or breast (females). It is recommended cats be neutered before sexual maturity which is usually nine to twelve months. Cats are a different case, so I'm not differing with Prajna stating dogs need neutering after sexual maturity.