We recently got a couple of guinea pigs as pets for the kids. Both were born in captivity (not the kids, the guinea pigs), in a cage at someone's house and have always been fed by humans, that is, they have never had to forage for their food.
Guinea pigs belong to the same family as mice and squirrels, but when it comes to being scared, they are closer to rabbits. The slightest alien noise sends them scurrying for cover.
When you drop their feed into the cage, usually leafy vegetables, they pick up one leaf and run away and hide some place and nibble furiously on it. It is the natural instinct of an animal that is small, defenceless and constantly in the danger of being gobbled up by a predator.
The increased life expectancy apart, I think turning forty and all its attendant fuss are also about a primeval instinct. It is biological rather than psychological. (Aren't most things?)
As I kid I once heard a distant uncle proclaim rather mysteriously, "After forty, you start realizing". Later in life, I heard variations that echoed the same thought. Of course, as a young man I would laugh it off as the justification of an old man for being old. After I turned forty, I began quoting the uncle (without attributing it to him of course). But I would explain the mystery if someone asked me. I would explain half-jokingly, "After forty, you start realizing: That you were a fool all these years and have done nothing but waste your time".
That is the key: Time.
Somewhere in our primeval brain, the instinct kicks in: You are forty. You don't have much time left. You are not going to be as healthy, as strong or as fertile. For women, it is an early warning signal that menopause is around the corner so go, have a go now while there is still time. For men, it must be about dwindling numbers (what's with men and numbers anyway?), so have a go while you can still count your blessings, in millions.
The same would then extend to our unfulfilled ambitions, desires, goals, whatever. Go write that story now! Begin to learn that language today!! Visit that destination this year!!! Pursue that skirt now!!!!!
That would also explain the regret about things undone, wanting to turn the clock back with botox, silicone, school/college reunions etc. etc.
Of course, this theory is based completely on my instinct.
But we know how strong instinct can be, don't we? I even have research done on guinea pigs (it can’t get more scientific than that!), to prove it.