I did some reading up, and Zeuterin is still only FDA approved for puppies aged 3 to 10 months. I read an article where the company that makes it is trying to get approval for dogs of all ages, but that hasn't happened yet. If you go to the manufacturer's web page, you will find lots of information. Apparently, the risk of adverse events is less than for traditional castration.
Non-surgical Neutering -- FAQ -- Ark Sciences ( Zinc Gluconate Neutralized by Arginine--FDA approved as Neutersol in 2003)