It's interesting that the similar "stretch of lifetime" is seen in Japanese ancient document (Kojiki), and probably in other cultures, too. Those ancient tales were inherited orally before recorded. Could it be that the storytellers tend to exaggerate? Or maybe counting the number of years wasn't that objective but had more subjective significance?

There is a theory that the age of Biblical patriarchs were originally recorded in months (969 months = 80 years and 9 months, which is quite reasonable) and somehow got mistranslated into years sometime later. This, however, doesn't quite work with Enoch who is said to have had a son at age 65.

So, maybe he had been married very young to an older female and had a, for him at the age of 5 a half years surprising, pregnancy attributed to him. His age of 365 days is still rather symbolic. His father's 81 years at that time are rather a lot. I'm not even sure that kind of arranged marriage was common. He was said to be living amongst sinners, anyhow, while his father was said to have had many children. The Grandfather of the family would have reached fatherhood at the same 65 (yoms, then), so, whatever.

Or maybe they were just stories that were taken at face value, as they have been for hundreds of years.

I don't know, if yoms has a different meaning, maybe father has, too.

I propose, children would be given responsibilities early on, they had to start soon. So, maybe being a father at figure 65 months means teaching a brother or cousin or other newborn and leaving the adults care for the food. They would form trust for one another while the resentment from biological parents towards their children, as shown with gods fury over adam and eve, would be mutual. "Alter" (older) in german can mean big brother, or father, but generally any subjectively old one.

Of course I mainly agree to the story aspect, but I don't know any other mysticism about the mentioned figures.