Why are old people cranky?

This is exciting.  Now I can vent to the world in general (that's if anyone ever reads it) instead of taking it out on my family and friends. 

Why are old people cranky?  I was lying in bed contemplating this the other night - after my usual 3 a.m. trip to the toilet after which I never seem to be able to get back to sleep easily, why is this?  Life is full of questions and I no longer have a parent to ask.  But to get back to the old and cranky question....the answer came to me.  I told my son the next day.  The reason old people are cranky is because they've been around long enough to recognise a fuckwit at 20 paces and they just shortcut the whole thing.  Go away!  I don't need to listen to your stupid comments.  I've heard them all before.  You're wrong and I don't have the time to waste telling you why. 

You ask an employee in the supermarket the whereabouts of an item.  Their eyes glaze over.  Clearly they don't know but are they going to say that?  Of course not!  They point vaguely away from where they are.  Aisle 12.  It's always a long way away.  I just know that I'm going to schlep all the way over there and find myself in the dog food aisle when what I'm looking for is soy milk.  By the time that happens the employee has safely made his escape and I'm left wandering around looking for somebody else to ask.   This makes me cranky.  So now I get cranky as soon as I see his eyes glaze over.  'You don't know, do you?'  'If you don't know you should just say so and I could find somebody who does know.'  He backs away from me.  Warning.  Warning.  Cranky old fart alert!