I had a job selling newspaper subscriptions door-to-door when I was young (like 15ish), the thing I noticed was that grumpy people are normally in the mid-30s to mid-50s range.
I worked that job for 9 months (it was awful) and I would talk to upwards of 30 people a night, 4-5 nights a week. What I noticed (and this is all anecdotal), 30's-mid 40's women were always very polite and would make time if they had it, men in that range were extremely rude and had little patience for a solicitor. Mid-40's to 60 the roles reversed, women would be a lot ruder, but the men would be nicer and almost had an appreciation that I was hustling and would make some time for me.
Over 60, most women and men were pretty laid back and accommodating if they had the time.
But I am sometimes surprised when people tell me that's not a universal feeling. At some point I came across some study claiming to show that it made people less trusting of people outside of family.
I took care of my mother until she passed away at 90. When she was young she never relaxed about anything. I think this was because of growing up during the great depression.
When she came to live with us she became very sweet and child like in her outlook. I think it was because she trusted us and felt she could relax. I feel sorry for my sisters because they only knew her middle years and never saw the person I saw.
I've seen the world change a lot in my years, and I can only imagine what it's like for someone who has lived two or even three times as long. Keep in mind they were born at a time when nearly half of the houses in America didn't have indoor plumbing.
Perhaps the grumpy ones stress themselves out and don't make it as long.
Or less morbidly, perhaps the grumpy ones are just not those you are likely to get to know.