This is going to be badly fragmented because I'm very tired, but possibly I'll have more brain tomorrow and be able to clarify.

I do not know how much this actually has to do with P&P, but there actually is a thing with 21 and 28. One of the ancient philosophers, Plato or Socrates I think, divided up the span of human lives into stages, each of which lasted 7 years. 0-7 infancy, 7-14 youth, and so on. This system is why 21 is a significant age. And 28 is the next marker after that. So that might be it.