What is the 120 minus age rule?

Could you elaborate on the "120 minus age rule" in the context of cryptocurrency investing? I've heard this mentioned in discussions about asset allocation, but I'm not entirely clear on its significance. Specifically, how does this rule help investors determine their allocation to high-risk, yet potentially high-reward assets like cryptocurrencies? Is it a general guideline or a strict mathematical formula? And how does it factor in other considerations like an individual's risk tolerance, financial goals, and investment horizon? I'd appreciate a concise yet thorough explanation of this concept.