I know I'll probably get this wrong, and not remember who proposed the theory, but I've always loved it.

The theory goes: we come from the great beyond (Wordsworth said we are born "trailing clouds of glory") and fall, as complete souls, into the physical body. The soul diminishes as the body grows and performs all that is required of it. Around middle age, as the body starts to diminish, the soul reawakens (we call it wisdom), and as the body deteriorates, the soul expands, preparing us again for the great beyond, to which we will return.

I didn't express it very well, I'm afraid. Heard it years ago; all I remember was it came from a very early Italian philosopher. I like the fact that there is compensation in middle and old life for our diminishing physicality.