I am told a boxwood only last 10 to 12 years. Mine have been in ground 11 years and are losing leaves in patches, look like dieting stops?
A common time frame that greenhouses or garden centers will give for the life of shrubbery in a landscape is 10 years.
Boxwood can live and thrive in a garden for many years with good growing conditions and care.
Boxwood have shallow root systems and need good draining soil.
Cold and winter conditions can effect the Boxwoods lifespan, as well as disease and insects.
Pruning your Boxwood is as much a part of their care as watering.
Here are several links that will help you.