Sheep live ten to tweleve years on on average but can live up to twenty years depending on their productivity. A sheeps health usually starts to decline at around age seven.
1 Additional Answer
Similar to breeds of dogs that are large, sheep can live anywhere from 10 to 20 years. However, 10 to 12 years is the average life expectancy. You can find more information here: http://www.sheep101.info/sheepbasics.html