To give you an idea: I was born well over four hundred years ago on the third of March 1581, in my parents’ room, on the third floor of a small French château that used to be my home. It was a warm day, apparently, for the time of year, and my mother had asked her nurse to open all the windows.

      ‘God smiled on you,’ my mother said. Though I think she might have added that – should He exist – the smile had been a frown ever since.

      My mother died a very long time ago. I, on the other hand, did not.

      You see, I have a condition.

      I thought of it as an illness for quite a while, but illness isn’t really the right word. Illness suggests sickness, and wasting away. Better to say I have a condition. A rare one, but not unique. One that no one knows about until they have it.

      It is not in any official medical journals. Nor does it go by an official name. The first respected doctor to give it one, back in the 1890s, called it ‘anageria’ with a soft ‘g’, but, for reasons that will become clear, that never became public knowledge.

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      The condition develops around puberty. What happens after that is, well, not much. Initially the ‘sufferer’ of the condition won’t notice they have it. After all, every day people wake up and see the same face they saw in the mirror yesterday. Day by day, week by week, even month by month, people don’t change in very perceptible ways.

      But as time goes by, at birthdays or other annual markers, people begin to notice you aren’t getting any older.

      The truth is, though, that the individual hasn’t stopped ageing. They age exactly the same way. Just much slower. The speed of ageing among those with anageria fluctuates a little, but generally it is a 1:15 ratio. Sometimes it is a year every thirteen or fourteen years but with me it is closer to fifteen.

      So, we are not immortal. Our minds and bodies aren’t in stasis. It’s just that, according to the latest, ever-changing science, various aspects of our ageing process – the molecular degeneration, the cross-linking between cells in a tissue, the cellular and molecular mutations (including, most significantly, to the nuclear DNA) – happen on another timeframe.

      My hair will go grey. I may go bald. Osteoarthritis and hearing loss are probable. My eyes are just as likely to suffer with age-related presbyopia. I will eventually lose muscle mass and mobility.

      A quirk of anageria is that it does tend to give you a heightened immune system, protecting you from many (not all) viral and bacterial infections, but ultimately even this begins to fade. Not to bore you with the science, but it seems our bone marrow produces more hematopoietic stem cells – the ones that lead to white blood cells – during our peak years, though it is important