Aaron,
In general large breeds usually have a pretty short life sadly. The books and experts say an average of 6 to 8 years for Danes, yet genetics play a huge role in the actual maximum age. It is true the Danes have an under sized heart, from what I've read, some people disagree. Our Dane was not the typical lethargic Dane, she was an athlete, to the point that she would drive us nuts sometimes. At 9 she still was a full on puppy, running 100 miles an hour all day every day. Rather than her heart, we lost her to a very aggressive inoperable tumor, it was wrapped through out her internal organs and was the size of a volley ball. To put that in to perspective, at 130 pounds, her chest cavity was the same size as an average adult person. She never once let on that there was anything wrong until it was to late.
Ironically, both of her parents are still alive at 14 and 15 years old. I bump into the breeder from time to time.
As Forrest would say, life is like a box of chocolates. ....