There are some interesting thoughts here about the symbolic dimensions of some of the numbers. That probably has to do with them on some level.
I classify these ages as a subset of the problem of large OT numbers. Such inflated or hyperbolic numbers cannot be given too restrictive an explanation because they occur in such varied circumstances. The number of Israelites that came out of Egypt would form a column so long it would stretch from the Nile Delta to the promised land. But look also at the weight of quail meat each Israelite was supposed to have gathered in Numbers. Or the weight of the crown David wore after the capture of Rabbah. Or the weight of Absalom’s hair.
The height of Goliath grows from 6-1/2 feet in the Dead Sea Scrolls (which indeed was a giant in a time of low protein consumption) to a little over seven feet in the LXX to over nine feet in the MT.
The wall of the minor Iron Age town of Aphek fell and supposedly killed ten times as many people as died in the collapse of the World Trade Center. There were single battles that are said to have killed more men than the US lost in WWII. So, this problem of unrealistic numbers has many expressions.
Rather than moving forward from early Genesis, look at it the other way around–as ages lengthening as we peer back in time. This may have conveyed to early readers a sense of “deep time,” of a dim past so full of years that they must be counted on a scale other than the one we employ for the affairs of daily life.
In a similar way, the Deluge in Genesis marked for the ancients something like the boundary we draw between civilization and prehistory or between the Neolithic and the Bronze Age. The comment in Genesis about the earth being filled with violence must have had a correspondence to collective memory, since one in ten skeletons from the Neolithic shows signs of violent death.
We have only speculation to go on in explaining the problem as a whole. The long ages in the Torah serve a spiritual purpose of some kind, if only to make clear that in the first chapters of Genesis we are in a cultural setting far removed from our own.