Answer and Explanation:
It's because of the idea that the biological age of clones and their chronological age is not synchronized and then, cloned animals die young.
When cloned, Dolly was conceived using a cell of a six-year-old sheep. And Dolly died at age six and a half, an early death for a race that lives at least nine years on average. People assumed that an adult's clone began its life with a disadvantage: instead of being a real "newborn", it seemed that the clone's internal age was earlier than his time existence indicated.
Although, it was shown that if we get the cell of an animal of any age and insert its nucleus into an unfertilized mature egg, we will obtain an individual who will be born with a completely renewed life expectancy. They confirm that the signs of biological and chronological aging of cloned sheep are the same as those of uncloned sheep. So the real reason of the dead of Dolly with its age its not because of premature aging.