"Do not act incautiously while confronting little bald wrinkly smiling men!"
Fundamental point. If you find yourself facing down against a very, very old man at any kind of competition, for example, kung fu, then it is not a particularly good plan to assume that his age implies he will be weak, useless, easy to beat etc. The fact that the man in question has most likely been practicing kung fu all his life, and that, more importantly, he is still alive, would seem to imply the exact opposite.
The quote is taken from Terry Pratchett's 26th Discworld novel, Thief of Time. The wrinkly man in question is a Monk of History named Lu-Tze, but other such men appear in the Discworld series, most notably Cohen the Barbarian and the other six members of his Silver Horde, who between them faced down and succesfully defeated an enemy army of roughly 700,000 men. None of the Horde were under the age of eighty.
The idea is not a new one. Similarly skilled old men also appear all over modern day movies, books, cartoon series, anime and comic books.
For completeness: Rule Two is "Never refuse a weapon", and Rule Three is merely a corollary of Rule One: "If you are holding a deadly weapon and you are facing an unarmed man in a pose of submission, be frightened."