How long is an age long, and why?

Part of the age issue is theological, which I don't think has attracted much attention.

Some thoughts:

Ancient (pagan -- or 'mimetic' and idealistic -- or exilic, such as Hinduism -- using Westphal's taxonomy) views of the age of the cosmos were of long ages ( far longer than modern), eternal existence or unquantified into myth. They all tended to dehistoricize the origin stories or myths (often accompanied by ludicrous and irrational causality and calling upon fantastical creatures -- perhaps related to pre-flood denizens?). That is, outside the flow of time we are in and thus unreachable by and uncontactable by us.

It is the 'uncontactable' that is important: they all thus tend to or actually de-personalize the creation event(s) and place the creating force at inaccessible remove from our life-world and historical flow. The 'gods' are incommunicado,  remote, and uninterested in the creature, except as fodder or slaves, it would seem

All of this serves to strand humanity in an impersonal cosmos with the only possible 'integration point' being, at best, fickle exploitative forces indifferent to man's good and often inimical to it. At worst enabling man's depravity, cruelty and assertion of determinative moral opinions 

Modern materialist 'long ages' touted as accommodating Evolutionary processes, but in fact far too short for NDE to produce the current biota, are in this league. As against the Genesis account (which extends across Genesis 1-3, IMO)  which places man for fellowship with God in the same time-space and historical flow that the stage of fellowship was created.

In this account God is communicatively with us and ontologically and actually present and active directly and personally in our time-space world, which he spoke (speaking the first gesture of fellowship) into existence. The creation is thus both historicized and personalized. It echos the love with which the Creator worked.

Furthermore, it grounds in what is truly real our epistemology, our ontology and our axiology, all in the God who is and who speaks, finally in his Messiah where the '2nd person of the godhead' takes on human form as the inheritor of the creation. The grounding is in our time-space shown in God's action in the days that we exist in without denigrating but rather exulting in them .They define the domain of his creatures who would reflect his image in his creation for them, in a resonant responsive relational congress.

So, modern 'long ages' push God's creative acts out of our sphere of action and history to de-couple the creation from our 'life-world' and intruding an existential barrier of an incomprehensible past unreflective of the warmth and care of the creation events. They make any interaction with God of dubious historical weight. This possibly invites back the Platonism which has dogged theology for too long and makes God personally differently from his revelation of self in Genesis 1-3. It detaches Messiah from the continuity of care (love) demonstrated in that account and fantasizes the New Creation, when the NT has it the palpable consummation of the Kingdom of Christ.