A couple of articles to get thoughts going:
(If you have other good articles for other sub-groups or aspects, sure, post 'em.)
As for me, I relate to the "in middle age" part, as I'm just too busy to have any sort of regular -- or irregular -- time to get together with a friend. Difference is, I don't really miss it, for now at least. I feel some unease as if I should miss it.
Reminds me of a poem I wrote several years ago - before I got married. I published it in the comments thread in "Dead Poets". The title is "The Empty Seat"
There’s an empty seat beside me; vacant by my choice.
I chose to live alone and free, with no companion voice.
I savored solitude and silence, unbound by another’s will.
But lately in the seat I sense a void that joy could fill.
There’s an empty seat beside me, though many there have taken rest.
And some clung to it dearly when I tired of it’s guest.
But I’d given my disclaimer when beauty settled there -
I‘d remind her who she knew I was, when she started the affair.
There’s an empty seat beside me that I dread when I come home.
Temples grayed and youth has fled, my selfishness outgrown.
Each night I retire to an empty bed and wonder - is it too late to meet
A partner I could love and wed, to fill the empty seat.