If a child is diagnosed with type 2 diabetes at a young age, will this decrease his lifespan?

Possibly. Patients with type 2 diabetes are at high risk for cardiovascular disease, kidney disease, nerve damage, and vision loss, among other things. It's essential that all patients with diabetes take their medication as directed, eat a healthy diet, and get exercise. Taking good care of diabetes will help the body stay healthier longer.
Doesn't have too. With support and a healthy lifestyle people with type 2 diabetes can life a full life and even reverse some of the Insulin resistance causing their disease (this is especially true in children!). Make a plan with your child about how to make some healthy changes in the family's routine; exercise every day and cook healthy meals at home. This works better if the whole family participates!
Life change required. T2dm is a marker that your child has a profile of feeding and or exercise activities that do not meet the needs of his/her body & should be changed.Failure to make real change in type of feeds & activity will likely result in: high blood pressure, kidney damage, changes or loss of vision, premature heart disease and death.Life change leading to loss of the extra weight can yield a normal life.