The model numbers dont have anything to do with age in general, they were supposed to refer to tht type of aircraft but when Boeing was feeling confident it would give its future jets bigger numbers to imply improvement or advancement but then when the newer jets started having issues they would adopt modifications of older naming schemes to reassure customers. Over the years that left the whole scheme in tatters.

It seems like no one really knows

https://en.wikipedia.org/wiki/Boeing_Commercial_Airplanes#Mo...