How old is John Evans' head balancer?

Excuse me, could you please clarify what you mean by "John Evans' head balancer"? It's an unusual phrase that doesn't immediately come across as a common term in finance or cryptocurrency. Are you referring to a specific tool, machine, or perhaps a metaphorical expression? If it's a technical term or device used in a particular field, I'd appreciate more context to understand its relevance and then I could try to provide an answer regarding its age. However, if it's a figurative phrase, it might not have a literal age associated with it.