This is not true. There are lots of myths about altering your dogs and it can get confusing trying to find information amidst the rumors and wives' tales. If you have a dog that is not altered, you should call your veterinarian and discuss having it done. Your veterinarian is your best source for advice and information regarding your pets. As I have been reminded, there are exceptions to every rule.
Pets are individuals and different breeds have different needs.
There are breeds that are predisposed to specific diseases and conditions that would only be aggravated or accelerated by neutering too young. For these reasons, it is important to be informed. Contact the breed club for your dog's breed and ask questions. They love there breeds and want the public to have the correct information concerning them. There are many breed specific sources of information online as well.