I've just returned from a trip I made to my parents' home to see my grandmother. She's 92, and had braved a long international journey to come to see her great-grandchildren for the first, and probably the last, time. My grandmother is a truly remarkable woman who led a truly remarkable life - she speaks five languages, lived on every continent bar Antarctica, made dozens of trips to Egypt as an amateur Egyptologist, and once mounted a canoe expedition up the Amazon river. And she had talent to match her sense of adventure - she was a great artist, winning many international prizes for her wood carvings. Most important to me, though, was the fact that she was the eccentric grandmother every child deserves to have. She'd seen enough and knew enough not to care what anyone else thought, living life absolutely (and sometimes infuriatingly) by her own rules. My fondest memories of childhood are of visits to her, filled with books, brilliant schemes, and the smell of sawdust in her studio.

The 8 years since I last saw her have been cruel. Physically, she seems almost unchanged, but her memory is fading fast. She loved playing with my baby son, but would often ask me who he was. I would re-explain that he was her great-grandson, and she would be delighted all over again. She was overwhelmed by the helter-skelter of family conversations and small children, and clearly yearned to be back home, where she spends her days in quiet reminiscence among the curios and collected oddities of 70 years of travel. Her days as an artist have long been over - she hasn't been able to hold a pencil for a decade, much less a wood-chisel or a jigsaw.

On a whim, a few days into the visit, I started up a sketch app on my iPad and and handed it to her. She was doubtful at first, but quickly became deeply engrossed. She sat for hours, hunched over, using the side of her index finger to draw. And draw. Saving a sketch and starting a new one was too complicated, so she used the eraser to clear space instead, drawing each image over the top of the previous one. At the end of the day she handed the iPad back to me with the final sketch of a beautiful, sad young woman still on the screen.

A self-portrait, she said with a smile.

Truly, the tragedy of life is not that we grow old, it's that we stay young.