I can think of two occasions that people I knew have died of heart attacks whilst playing football. The first one was back in the 70s. I was 11 and in middle school, a boy in the year below me was playing football at break time and died on the field. Turned out that he had a hole in the heart. He was 10.
Years later the local milkman was playing football for a local team in the park behind my house. He dropped dead in the middle of the game. He was 32 and had three small kids.