Why are old people grumpy?

That's a very stereotypical thing to say. Not all old people are grumpy.
But obviously some are, but then again some young people can be too.

But before you judge an elderly person as 'grumpy' remember that they may have something wrong with them - they may be in a lot of pain, or they may even have a mental affliction such as alzheimers or dementia - I'm pretty sure both of these disorders cause mood swings.

They are probably so filled with overkill from their lives and senseless things they've been told.