Compared to other killers from a public health standpoint, ADHD is bad. Smoking, for example, reduces life expectancy by 2.4 years, and if you smoke more than 20 cigarettes a day you’re down about 6.5 years. For diabetes and obesity it’s a couple of years. For elevated blood cholesterol, it’s 9 months. ADHD is worse than the top 5 killers in the U.S. combined.

Having ADHD costs a person nearly thirteen years of life, on average. Barkley adds, And that’s on top of all the findings of a greater risk for accidental injury and suicide…About two-thirds of people with ADHD have a life expectancy reduced by up to 21 years.

This is from Edward M. Hallowell and John J. Ratey in their book ADHD 2.0

Here is some more background on the research