Mandatory Spay/Neuter

This is a tough one for me. Most of my dogs have been rescues, and I understand the need to keep the unhomed pet population from growing. But I also understand the physiological and perhaps psychological effects of early spay and neuter.
Los Angeles recently passed a mandatory spay/neuter law, and I understand that they may want animals altered as young as four months. While this has been fairly common with shelters doing adoptions - insisting that the animal be spayed or neutered before being adopted out - it hasn't been common among dog owners who obtain their canines from other sources.
So how young is too young? Well, four months just might be too young. At a veterinary conference I attended recently, the presenter gave the pros and cons of neutering earlier and later, and concluded that six months, as common wisdom has long held, is probably the correct age for altering a dog.
Yes, spaying before the first heat does greatly reduce the risk of mammary cancer. And that's certainly a terrific benefit. But it also may make females prone to urinary incontinence later in life. And the closure of the growth plates in the long bones (in this case, the dog's legs) is regulated by the normal hormones, which aren't present in the usual quantities if the animal is spayed. The effect on bone growth may be even more pronounced in male dogs.
So this is a balancing act. Shelters certainly have a strong impetus to be sure that the animals they adopt out can't add to the population. But for others to be forced to spay or neuter earlier than the best medical knowledge may indicate, well, that's just bad legislation.
And just in case someone from the Los Angeles city council happens by to read this, I would like to say that mandatory spay/neuter has not yet solved an animal population problem. The same people who ignore leash laws, license laws, and general animal welfare will ignore the spay/neuter laws. But you're probably going to make a lot of reponsible dog owners unhappy.