There are actually a lot of causes. Barkley (2018)[1] says:
> The reduced ELE linked to ADHD was found to
be a function of the first order variables of less education, less
annual income, greater consumption of alcohol and tobacco,
diminished sleep, and poorer overall health status relative to
the control group. Moreover, ELE was also shown to be a
function of the second order traits of deficient behavioral inhi-
bition in daily life and, much less so, of low verbal IQ, greater
interpersonal hostility, and deficient nonverbal fluency.
and:
> For instance, besides ADHD-C itself, this study showed
that behavioral disinhibition (as assessed by EF ratings in
daily life), verbal IQ, the comorbid psychopathology of
hostility, and the EF of nonverbal fluency (and its associa-
tion with nonverbal working memory) all uniquely contrib-
uted, in descending order, to variation in life expectancy.
The largest percentage, however, by far was contributed by
behavioral disinhibition.
Nigg (2013)[2] says:
> ADHD is clearly associated with worse physical health outcomes across multiple domains. These include unhealthy behaviors and outcomes like smoking, illicit substance use disorders, accidental injuries, obesity, and even suicide attempts and completed suicides.
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Basically, having ADHD means you have less self-control, engage in riskier behaviour (including riskier eating habits, substance abuse, driving, sex. It also means you struggle in life, resulting in suicidality, aggressiveness and anti-social behaviour. You sleep worse. You're more likely to smoke because nicotine is a stimulant which has a medicating effect on ADHD symptoms.
[1]: Hyperactive Child Syndrome and Estimated Life Expectancy at Young Adult Follow-Up: The Role of ADHD Persistence and Other Potential Predictors, Barkley 2018
[2]: Attention-deficit/hyperactivity disorder and adverse health outcomes, Nigg 2013 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4322430/)