Ways that being autistic can lead to premature death:
Stress and mental health related physical illnesses - hypertension leading to heart disease and stroke, lowered immunity, auto-immune diseases, increased risk of accidents due to cognitive difficulties and inattention.
Drug and alcohol abuse.
Inability to recognise when we're ill, including altered pain sensations and the fact that we struggle to interact with others so avoid seeking medical help.
Difficulty accessing medical care. I've lost count of the number of health professionals I've inadvertently pissed off due to my autism. They just consider me weird and defensive and don't want to deal with me.
Not being able to earn enough in order to lead a healthy lifestyle. Not having proper food to eat, heating, housing etc.
Mental illness resulting from alienation from support networks, chronic loneliness and being a societal outcast - can lead to suicide.
Side effects from psychoactive drugs that we're prescribed in order to help us appear normal to the neurotypicals cope in the world we live in. Iatrogenic disease caused by these drugs.
Chronic bullying can lead to mental illness and suicide.
These are just off the top of my head.