Obviously dogs aren't horses, but I think pretty much any equestrian can confirm that geldings (castrated horses) live as long or longer than stallions, in as good of health or better. I'd be interested to hear what specifically is different about canine physiology that being neutered when full-grown would cause health problems in dogs where it doesn't in equines.
I've yet to see scientific evidence showing, or even strongly suggesting, that neutering full-grown dogs has a deleterious effect on their health or longevity. If there's been a study or some such that I'm not aware of, I'd be very interested to hear about that, as well.
Heck, there have been a few interesting historical studies that suggest eunuchs probably had longer lifespans that intact men who had similar lifestyles...so that's something to think about. What would you trade for some extra years of life? lol.
I have seen studies on gelding young vs older. The research shows there was about 1/4" size difference. Hardly enough to worry about.
All my dogs have been altered fairly young. We adopted one dog that was neutered shortly before we adopted him at 2 years old. We lost him to cancer at 8 years old.
We have a female that was spayed during her first heat cycle (emergency spay, as she prolapsed), she'll be 15 in a few months, and other than being hard of hearing, she's super healthy.
As with question like this, you're going to get a WIDE range of answers, and everyone thinks they are right.
For me, I typically adopt my dogs so they are altered when I get them. I also compete in AKC agility, rally and obedience, so they have to be altered.
I start them in the rally ring to get them ring experience, while waiting for them to be old enough to compete in agility. My current agility dog was spayed when I got her at 12 weeks old. She'll be 9 this year, and was just diagnosed with hip dyplasia....but we think she is a Lab/Boxer/Golden Retriever, and those breeds are all prone to it. Chances are very high that she would have it no matter what age she was spayed.
Bottom line. You have to make the decision that you are comfortable with. Dogs have been altered at young ages for a long time, and dogs have been left intact their whole lives. Some issues in both instances.