Old people are pissed off and want you to know it. They yell at you to get off their lawn and get a damned haircut when you're across the street, wearing a hat. They seem to constantly be venting hundreds of years of pent up, decrepit rage.
But the reason goes beyond, "Being old sucks and they're mad about it." Most misdirected oldster rage comes from fear that results from their vestigial "fight or flight" mechanism. Nothing makes a person more fearful than seeing their bodily functions slowly shut down before their eyes, and there isn't exactly a way to escape from it. So "fight" is all that's left.
I'd kick my own ass if it hadn't disappeared five years ago.
It doesn't help that today's old-folks were raised at a time when it wasn't considered cool to talk about your problems in any kind of constructive way. You sucked it up and lived with it. If you committed suicide, they would literally call you a fag in the obituary. Well, if you "suck it up" for 80 years it eventually just overflows onto everyone who walks past your house.
So despite how thrilling retirement sounds when you're 24 and planning on spending every waking moment of it drunk and naked in a kiddie pool; for elderly folks who wake up seven times a night to go to the bathroom, hobble around with arthritis and spend half their social security on food for a cat that pisses on all their clothes (see #5), retirement can be a long, drawn out frustration of building tension with no release and no control.
Though, even the source we linked above acknowledges that in some cases they were dicks before they got old.
So you're a kid, and your decrepit Great Aunt shows up in her nearly purple hair and a massive floral-print dress. As her support hose bunches around her ankles like the wrinkly legs of an elephant and she complains about another bout with the rheumatism, she'll arch her crackly back, lean forward to plant a kiss on your chubby, scared shitless cheek, and spread her massive maw, painted in classic Whore Red, to reveal quite the same shade smeared across her dentures as though this morning she had opted to have lipstick for breakfast rather than simply apply it as is customary.
What you may not have known at the time is that Granny Red Tooth wasn't necessarily insane or trying to start a new fashion trend, but is instead suffering from any number of neurological disorders characterized by shakes and spasms making it remarkably difficult for her to manage the application of that insanely inappropriate shade near her choppers.
According to research, over 10 percent of people over 65 suffer some manner of shaking. That means tasks as simple as the application of make up, eating with a fork or even throwing a ninja star accurately can be more of a chore than you'd think.
Combine that with eyesight problems beyond what can be fixed with a simple pair of old lady glasses, and it's easy to imagine how you can leave the bathroom looking like a bizarre combination of the Joker, and someone who just ate the Joker.