A co-worker of mine, male 38. Healthy as hell. Never smoked, drank moderately , jogged every day at work. Was playing hockey one night, had a heart attack and died on the ice.
I’ve known a few who had a form of Muscular Dystrophy who’ve died comparatively young. (mid 20s)
My mother dropped dead of a Massive Stroke. She was in her early 60’s and a heavy smoker. She would not go to a Dr. so no one knew that she was (or was not) sick. I have known a few people that have had embolisms and died suddenly. I have known approximately 10 people that have “dropped dead”.
back in the late 70s, a girl i knew fared a bit better in a similar situation. i found her all but unconscious about 4am in the womens’ bathroom on the fourth floor of menk hall at ball state university.
she’d collapsed from toxic shock syndrome and was halfway to dead when i found her. she recovered without any troubles, thank god.
a high school friend of mine died in her sleep of heart complications. alive one day, dead the next. that was a shocker, to say the least. not only was it the first person i lost that i knew personally, but i’d say it was really the first time i truly realized i wasn’t going to live forever. 
I’ve known three.
- A fifth-grade classmate who died of leukemia.
- A high-school classmate (age 16) who died of a severe food allergy. (She was eating out at a restaurant with her grandmother, and the allergic reaction set in so quickly she died before they could get her to the hospital.)
- A former high-school classmate and fellow freshman chemistry major at Iowa State, who died of pneumonia during our first college summer break. He would have been around age 19 when he passed.
Guy I went to high school with, age 17. We started our senior year, and he was fine. He suddenly started having weird behavior problems; violent outbursts, inappropriate language, just totally not himself. Turned out he had a brain tumor, some particularly aggressive type. He didn’t come back after Thanksgiving break, and died after the first of the year.
A friend of mine (age 32) woke up one morning, complained to her husband of a horrible headache and just collapsed. She’d had an aneurysm that burst and was probably dead before she hit the floor. It was one of those things that she’d had all her life, but never known about.
A friend from college (age 19) had an epileptic seizure and hit her head on the sidewalk.
Former co-worker died in his early 20s of an undiagnosed heart problem. His twin brother got checked out afterwards and found he had the same thing.
My former babysitter died of heat stroke at age 19. I was shocked by it. She had seemed immortal to me, as are all teens.
A classmate was murdered by a robber at his gas-station job. He was 16.
My second-cousin’s child died recently but it was not unexpected as he had congenital health issues. He was only 7.
My best friend from HS’s younger sister died less than a year after she graduated HS, just months before her 19th birthday from a brain aneurysm. She had complained of bad headaches for a few days before. She went to sleep one night and never woke up.
When I was a junior in high school two of my classmates died suddenly. There were whispers of suicide, but nothing ever verified.
We’d had a third classmate who died of cancer when she was 15.
Snap. If yours was in Edinburgh around '93 maybe we’re even thinking of the same person
Not suddenly, but the neighbor and friend of my high school best friend was a girl of twenty who died of kidney failure.
My next door neighbor’s 19 year old son went from fine to dead from viral encephalitis in the space of just over 24 hours. This was 2006 or so.
We lost a kid to some oddball cancer when I was in high school back in the 70s.
I almost remember something about a similar medical case back in my elementary school days, but that might have been a car accident.
A friend of mine fell victim to a heart condition while on a work-study program in New Zealand, and while he was eventually cleared to travel home to Ontario, he only survived a little more than 24 hours after coming home. He was 19.
A girl who graduated with me who was in the pink of health, had a good boyfriend, and a full scholarship to college, was nonetheless picking up some extra money working on a moving crew over the summer. Even though the work was hard she really enjoyed the job, and believe me there was no kid I graduated with who was set up to succeed in life.
One day just a couple of weeks before it was time to quit and start college, working on a crew moving a family into their new home, everyone on the crew sat around the garage in the shade and took a break to have some sodas. According to the witnesses, she sat back against an appliance that had just been moved into the garage, jerked suddenly, and then seemed to go to sleep.
A couple of minutes later someone went over to wake her up, and got the shit shocked out of him. Long story short, she was dead - the appliance body, through some freakish accident which likely happened during the banging of the moving, had become shorted (it was 220V, FTR), and had no other ground, and when she sat back against it it electrocuted her. She made no sound, and gave no indication other than the jerk, and died right in front of everyone, and they never knew.
It still makes me feel sad.
…although I guess he kinda goes against the “not from accident” specified by the OP.
My cousin died unexpectedly in her sleep. She was autopsied but cause of death remains unknown. Her funeral was on her 16th birthday.
A school friend of mine had had random symptoms (like fever and various pains) for a couple of years, but was never diagnosed with anything. Her family and psychiatrist believed she was really sick, but her regular doctors had come to believe it was all in her head. She collapsed and died shortly before her 15th birthday. Nobody ever knew why.
The wife of one of my husband’s friends died a year ago at the age of 31. She was reading in bed with her husband. He thought she was rolling over to go to sleep, but she died right in front of him. I think it was a heart thing.
Too old, maybe, but my sister’s brother-in-law died unexpectedly in his sleep a few days ago. He was 50.
When I was in high school we had a Junior die every year for 3 years. It seemed like a curse. One was a car wreck (drunk driver hit them 
),I forget the next one, and in my class the girl stood up to give a speech, which popped a large brain aneurysm in her head no one knew about. She died instantly.