A FEW YEARS AGO I WAS AT OUR HUMANE LEAUGE AND THERE WAS A SAINT BERAND PUPPY THAT THEY SAID WAS 6 MONTHS OF AGE. AND SINCE I HAVE ALWAYS LIKED MASSIVE BREEDS I FEEL IN LOVE WITH HIM. THAN WE BROUGHT HIM INTO OUR HOME AND HE STOLED MY HEART I EVEN QUIT MY JOB SO HE WOULDN'T HAVE TO BE ALONE. AND ONE DAY HE TOOK SICK AND CRIED LIKE I NEVER HEARD A DOG SCREAM AND I TOOK HIM TO OUR FAMILY VET AT 4 IN THE MORNING AND AFTER ONLY HAVEING HIM A MATTER OF MONTHS ON THAT EVENING WE WERE TOLD THE BEST THING WE COULD DO FOR HIM WAS PUT HIM TO SLEEP WELL IT TOOK ME FOREVER TO GET OVER THE LOSS OF HIM. AND MY FAMILY WENT WITHOUT ME KNOWING TO A BREEDER THEY FOUND AND SURPRISED ME WITH NEPTUNE. HE IS NOW 16 MONTHS OLD AND HE WEIGHS 180 LBS AND HES THE LOVE OF MY LIFE THAN I READ SOMETHING ON A WEB SITE THAT SAID THEY ONLY LIVE TILL AROUND 7 YEARS OLD COULD SOMEONE TELL ME WHY THAT IS? I UNDERSTOOD FROM MY VET THAT THE REASON MY FIRST BABY DIED WAS DUE TO HIM NOT BEING FEED CORRECTLY BEFORE I GOT HIM AND HIS STOMACH TWISTED AND I WANT TO KNOW CAN THAT HAPPEN TO MY NEW LOVE AND ALSO WHY IS THERE LIFE SPAND ONLY 7 IS THAT CORRECT OR WRONG INFO?          THANK YOU..ANNETE.

The stomach twisting thing is called torsion - it's not a chronic condition, it happens in the short term and is often fatal.  the most important way of avoiding it is to keep stress levels in the dog very low, particularly after feeding, and to not excercise directly after feeding.
The giant breeds are known for not having the long lifespan of smaller dogs, 7 years is about average, but they can live to 9....

My friend has a huge Great Dane that is almost 14. With good  overall management and being  kept healthy all her Danes have exceeded the average lifespan of 8 years or so. Chin up !! there is no reason for you to think your Saint will automatically die at 7 he may live alot longer. Just do things correctly for him and I am sure you will have him longer than you think!.

As the others say - much of it is up to you. Feed him correctly, give him the right sort of exercise & he'll have a big advantage. Sadly, it's usually true that the bigger & heavier the dog the shorter the lifespan. Hey, he's only a youngster, don't spoil your time together worrying about what might happen - enjoy him, make the most of your time together. When the time does come you'll know he had the very best of everything and that you couldn't have done any more. Unfortunately, it's something we all have to accept (and it's very hard), whether they live for 7 or 17 years.