Intolerable cruelty

I don't understand why old people become cranky bitter crackpots when they're old. Is it because they have it against the world and blames everyone that they don't have their youth anymore? Recently I noticed my mom has become progressively worse in her behaviour, i.e. her nagging and doom-and-gloom outlook on life, in fact nowadays she's starting to swear a lot! It worries me because she seems like a far cry from the woman I used to know when I was a child, I mean she never was like a sweet young thing but still, she had a brighter outlook on things, nowadays talking to her just makes me think that the world is going to end anytime. It's scary and it seems that most of my friends around my age are going through the same thing with their moms! What is it that makes old people just go all cucko when they reach a certain age?!?