I'm old, and I've poured over the changing research since before the internet. The push for spay/neuter by vets and anyone, actually, bothers me. There is absolutely no hard and fast rule. (Though suggesting you should even consider not spaying so you can make money being a backyard breeder is really disturbing for responsible owners.) There are health pros and cons and the bottom line there are life-threatening dangers either way. As for age, mammary cancer is almost zero if you spay before the first heat, still greatly reduced before 2nd, and no difference if you let her have 2 heats. You can help by checking each nipple each month (8 on the 8 -- dogs generally have 8 nipples, so they started the idea based on that). Even found early, though, it can still be a death sentence. BUT there are also growth plate and other issues, even temperament, with early spaying (prior to a year or 2). No one but you gets to decide which way you want to go because it will be you holding her if the choice turns out to be bad for her. And you won't know unless it happens.
Some breeds (Samoyeds being one of them) have high rates of diabetes in females who aren't spayed prior to first heat.
There are non-health issues. Can you absolutely keep male dogs away from her when she is in heat? Do you live in a house with children or someone who cannot be trusted to keep her safe? Are you willing to deal with neighbors' howling male dogs (though there are sprays for where she pees as well as wipes to help keep the hormone smell from broadcasting her status) and roaming loose dogs visiting your yard?
Too many vets seem programmed to believe clients are idiots. They said yearly vaccines even though proven unnecessary because owners wouldn't get health checks otherwise. They push spay/neuter because we're too stupid to keep our dogs from mating. Talk to your vet, but make your own decision.
I admit that the older I am the less I believe in spay/neuter. But that's for my own situation and abilities.
