I have not read the research behind the published statements. I tend to discount people who make extreme assertions, such as saying that early school entry can cause premature death. I think there is a positive correlation between parental income and child life expectancy (I know that correlation between a person's income and his own life expectancy is positive), and there is a positive correlation between educational attainment and longevity. It would be surprising if gifted children from affluent families have lower life expectancy.

Doesn't letting a child "develop naturally" mean not slowing them down?

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Pupils should not be subjected to full classroom tuition until the age of six to off-set the effects of premature “adultification”, it was claimed.
Dr Richard House, a senior lecturer at Roehampton University’s Research Centre for Therapeutic Education, said gifted pupils from relatively affluent backgrounds suffered the most from being pushed “too far, too fast”.
He quoted a major US study – carried out over eight decades – that showed children’s “run-away intellect” actually benefited from being slowed down in the early years, allowing them to develop naturally.
Many bright children can grow up in an “intellectually unbalanced way”, suffering lifelong negative health effects and even premature death, after being pushed into formal schooling too quickly, he said.