Another interesting post amongst the mini zeitgeist I'm currently working in can be found at the Pits Perilous blog by Olde House Rules (find the post here). It delves into the idea of long lived characters such as elves, and to a lesser extent dwarves, compared to traditionally shorter lived races such as humans.
The article can instantly be seen as analogous to my dilemma of using player character spirits and familiars who run the gamut from a infant spritelings couple of months old through to immortal forces of nature for whom the entirety of recorded history has been the blink of an eye.
It proposes an elegant solution, where all races reach maturity at much the same pace, then diverge once adulthood is attained. I've proposed similar ideas in the past, where human genetics sees cell degradation gradually accelerate (thus causing aging), while the cell degradation of other races occurs at differing rates (thus accounting for their varied lifespans while seeing basic maturity manifest at roughly the same age). But for spirit beings, who have chronologically been "alive" for exponentially different timeframes, this doesn't really cut it.
I guess it goes back to the concept of what experience is. I've already decided that experience does not equate to age, but rather is proportional to the activity of the individual, and the risks they have taken. Existing in the real world is a risky activity, but it provides knowledge and power about maintaining an ongoing existence. Observing the world from up close may also provide knowledge, but it's less visceral, so the accumulation of data is a slower process. Observing from afar is safer still, but only really grants macroscopic data of the widest trends. It then seems easy for us to tie a rate of spiritual aging, to the closeness of that spirit to reality. Those who manifest in "meat-space" age and develop at the rate of the mortals. Those who linger in the penumbral periphery might age at a fraction of that rate (for argument, lets say a tenth)... here they exist on the edge of a mortal's vision, they can observe but not interact. Those who exist in deeper planes may be able to observe the penumbra, but they cannot see anything of the material-plane/"meatspace" beyond its ripples through the spirit realms (to continue the analogy, such beings might age at a hundredth of the mortal rate). Beings further detached from reality start to lose their point of reference to mortals, they need to fracture their essence to create avatars capable of drawing closer to the mortal realm, or spawn angels, demigods and other lesser spirits to act as intermediaries (such beings would age at a thousandth of the mortal rate, or slower still if they existed even further from the physical).
If an average mortal lifespan is 70-100 years, we suddenly have commonly encountered spirits capable of living several centuries to a millenium... distant spirits capable of living several millennia... and alien beings observing on the edge of reality who could easily watch ice ages come and go. But the longer the lifespan, the more alien and exotic they are, and the less they are able to meaningfully interact with the mortal world. There's the balance.
Similarly, we get periods of activity or periods of slumber/torpor/inactivity. I'm seeing a lot of spirits functioning in the Dark Places as drones. As such, they perform a simple duty in the spirit realm that supports the structure of the mortal realm, but it is when they break from this drone activity that things get interesting. These are the stories we tell. Most characters will have a few years of meaningful experiences to draw on, the youngest ones might be in contact with an Akashic record, or retain fragmentary knowledge from the greater spirit who spawned them.
I like where this is heading, but there's more work to do.