The condition Dystonia itself is not fatal and you do not die of dystonia directly.
But Dystonia patients may die soon after since Dystonia is often the result of other diseases that affect mortality and affect the persons lifespan.
So the person that dystonia is affecting could die from other diseases that lead up to dystonia.
The dystonia doesn't shorten the lifespan but the other diseases that the person has before dystonia can cause death.
Some people who do get dystonia do die within 5 to 10 years after the dystonia diagnoses but it's because of the other diseases as well as the persons age at time of the dystonia diagnoses.