The 25 yo lieutenant I mentioned above died of a massive coronary and a 35 yo major I knew barely survived one. These were military members in very good shape.
So yes, it does happen, even to young people.
When I lived in TX, my 37 year old neighbor dropped dead on his front lawn a week after he’d had a complete executive physical exam which he’d passed with flying colors.
Those of us who knew him really well all knew that day was coming, though. I'd imagine people who were really close to Breitbart weren't as shocked as the rest of us are now.