I have a Newfoundland, and my breeder asked us to wait until she was about a year old, or after her first heat cycle but before her 2nd. For males she suggests no earlier than 12 months and possibly waiting till 18-24 months.
There are studies ( although not "approved") that show waiting helps with their growth plates. Something about the hormones, etc. I know a few people with Newfs who had them done early and now have bad hips, etc. A lot of that also has to do with their parents, so my next question would be were the parents tested? My puppy's parents were in the 90th percentile, so while that means she's got a pretty good shot at not having issues, she still could. It's just part of owning a giant breed. The studies also suggest there isn't a big difference in the amount of cancers and issues between neutered/spayed dogs and those who weren't.
My main concern if we had got a male would be agression, and what he would do if he smelled a female in heat. Some Newfs top out at 180, and they are super strong.
I would also suggest finding a vet who knows and specializes in giant breeds. They can have a host of issues little dogs can't, like bloat, wobblers, dysplasia and ACL tears. Danes especially are more likely to bloat than other giant breeds, I beleive.