Three-quarters of the way through the diary that Jan Morris started at the age of 90, a book by turns fanciful, funny and spiritual, Morris becomes momentarily morose. She has woken after a bad night to ponder “the miseries of nonagenarianism”. At last, it seems the book will behave its age. In an era when more people live to be 90 and older and, in the case of Malaysia’s Mahathir Muhammad and the Queen of England, even carry out their executive or ceremonial duties much as before, a book by one of the world’s most celebrated non-fiction writers on navigating

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