Without even knowing it aging brings an element of narcissism. So many of the young see their elders as selfish, and yet they, in turn think the same about the young. Maybe getting older brings the inevitability of grumpiness, and yet aren't teenagers grumpy too? Developmentally we pass through milestones and repeat the traits of youth in old age. However becoming grumpy is also about self protection, many do not want to be seen as helpless and lonely so maybe the irritability is about self survival.
Not all are grumpy all the time. I, for example, like to see myself as a trifle eccentric rather than grumpy, and secretly like the idea of being outspoken and different and I can see that behaviour growing as I move further on in life. Mind, some might say worsening rather than growing, it all depends on your point of view.
As we age we can become guilty of getting into cliques within our own set. Recently I joined a group of people who had been together on weekly basis and sat down having been told I could sit anywhere by the leader, only then was it pionted out "You can't sit there it's Betty's chair". It's disconcerting when you realise you might have done the same thing to someone else. I later felt petty, mean and withholding but also realised that sitting in the same chair is a form of security, one sometimes has to work hard to become a fully fledged member of the so called clique.
In some ways twitter is a clique, gathering followers, feeling happier as the numbers grow, finding yourself fitting in slowly. Oh dear! it happens to the nicest people. It feels some times as if one can't have a day off in case they forget about you. How sad is that?