Most centenarians attribute their great age to some magic elixir or other. The longevity of the Italian scientist Rita Levi-Montalcini, who this week became the first Nobel Prize-winner to reach the age of 100, might be the result of a potion that is a little out of the ordinary: Professor Levi-Montalcini, it is said, puts her undiminished mental vigour down to regular doses of nerve growth factor (NGF) – the discovery that made her famous.