I have seen Jacinda Ardern described as New Zealand's youngest ever Prime Minister. This is strictly true, but she is not New Zealand's youngest ever head of government. That honour goes to Edward Stafford, who was 37 years and 40 days old in 1856, when he became the country's third Colonial Secretary. Jacinda Ardern was 37 years and 92 days old when she became Prime Minister.
(It was in 1856 that New Zealand became responsible to the Westminster parliament, rather than part of New South Wales and responsible to the monarch. Its heads of government were known as Colonial Secretaries from 1856 to 1869, Premiers from 1869 to 1907, and Prime Ministers from 1907. According to Wikipedia, Edward Stafford's two predecessors as Colonial Secretary lasted only 14 days each: Henry Sewell from the 7th to the 20th of May 1856, and Sir William Fox from 20 May to 2 June. I make that 13 days each, but we'll let that pass.)
At the time of her appointment, Jacinda Ardern was the world's youngest female head of government. She lost that title in 2019, when Sanna Marin became Prime Minister of Finland aged 34 years and 24 days.
© Haydn Thompson 2020