What’s the best age to neuter or spay medium to large sized mixed breed pups?

There is all kinds of thoughts about this & we even argue among ourselves as to when to spay/neuter a dog.  I have always thought that getting them spayed/neutered prior to puberty was the way to go. So that says that at least by the age of 6 months.  I had a couple of pups 4 months old spayed/neutered.  I don't want a male to be marking & neutering prior to the testosterone helps with that.  But the main ideas is to stop any kind of reproduction.  Spaying/neutering helps keep the pet population somewhat in check.

A lot would depend, with bitches, on whether you can be sure to keep her safe during her first season, so she doesn't end up being a puppy producing puppies!   However, having said that, and I have personal experience of this with my Whippet when I approached my then vet to have her spayed before her first season (because I had an enderly entire male with her) and he persuaded me to wait until after her first season, citing bone development problems and the potential for spay incontinence if done early.   I agreed but as there was no sign of a season by around 11 months, asked him to go ahead.   He did and she's fine.

In general however, if castration for a male is needed at all, I'd not have that done before the growth plates have closed - certainly not before a year of age.  I might change my mind with a small breed.

Bottom line.   Not before a year of age, to allow the growth plates to close.

Five to six months avoids reproductive cancers, later in life. Ask the vet who suggests after first heat, the reason to wait. Ask about reproductive cancers from both vets. We spayed at 6 months and our dogs lived to about 17 years of age. Larger dogs. You might also consult with the Humane Society and ask them about this.

Of course, normal vet care is in order to maintain general health.

The vet you are using is the person to ask.... however it will depend on when the vet trained, decades ago or more recently as like anyone else other training, new methods , new thinking, new ideas.....fact is it is up to you to decide and you can keep on asking other people.other vets until you get the answer you want.....

I personally do once the dog is mature and maturity depends on the dog, you 'know' it has matured if it has had a season, but I would get it done just before that first season it is better long term healthwise.... you trust the vet you use or you don't, you ask them questions and  you challenge their thinking/beleiving... vets are people, they are not the be all and end all for advice, many are dog owners themselves and they don't do for their own dogs what they recommend to clients .... and I say that as two of my long term friends are vets

i would just do what those vets say

About 6 months.