I've read both threads now and I think they are about the same thing...
Anger. Probably linked to low testosterone levels, but made worse by such things as:
defeat in the egg and spoon race on school sports day, a lowly examination grade, critical parents, a menial job where efforts went unappreciated, a missed promotion, jilted lovers, a partnership that went septic, financial jeopardy, unreliable friends, an estranged family, fading health, a feeling of a lack of personal fulfilment.
There was a good article I read a while ago and it said there are 7 ways to spot if you are becoming a grumpy old person:
1. You refuse to make new friends
2. Clothes exist for function not fashion
3. Technology refuses to co-operate with you
4. You have replaced conversation with complaints
5. Leaving the house becomes unnecessary trouble
6. You baulk at the "so-called" evolution of language
7. You just don't care
It comes to all of us does old age. I'm a young whippersnapper in my early 60's. I can still run, cycle, weight-train down the gym, climb trees, play conkers, play football, have sex several times a week (sometimes including other people in it).
I saw my eldest brother start to turn into an old man about 10 years ago now. He stopped listening to pop music. He stopped wearing fashionable clothes. He lost interest in travelling abroad. He bought a caravan. And eventually...he started playing crown green bowls.

(death knell imo!).
This was the start of it.
After that, it wasn't long before he started wearing cardigans and corduroy trousers.
He stopped going out to restaurants for meals.
Everything became a "rip off" and a "bloody racket".
He became very short tempered and irritable about the slightest thing. He seemed happiest when arguing. These are all signs that the chemical that gave you splendid boners in your younger years, but has since made your hair fall out, is now disappearing at quite a rate and without it you are turning into a grumpster!
His wife, God bless her, has learned that the best thing to do with him is just ignore him when he's like that - and occasionally he'll snap out of it and become the nice person we all know he is, for a short while anyhow! But more often than not, he'll find something to drag up, to try and salvage some kind of victory over his perceived adversaries. And, like a dog with a bone, he will not let it go. He'll shake it and rip it and snarl at it, until it is completely destroyed.