Further to my earlier post regarding the significance of the number twenty-one in the game, an Internet search reveals the following quote from E. Phillips Oppenheim's 1914 novel 'The Vanished Messenger':
"I came," he went on, "because it occurred to me that, after all, I had my duties as your guardian, dear Esther. I am not sure we can permit flirtations, you know. Let me see, how old are you?"
"Twenty-one," she replied.
Is this merely a great coincidence, or is the game a nod to the mystery/crime author?