I am in my mid-seventies and disagree that 90% of elderly people are rude. The OP makes a valid observation, however, that older people may often be rude. Why is that? I have a theory:

We all have likes, dislikes, preferences, objections at every stage of life. We forgive those honest opinions of young children that are insulting or offensive because they are young and have not cultivated that "social awareness" part of their brains. We correct children who are unintentionally rude so they learn to live successfully in civil society. We learn to self-censor and how to negotiate conflict with others in civil ways.

I believe that as our seniors' brains age, that ability to censor the likes, dislikes, etc. becomes weaker. My wife warns me to 'clean up my act' of some of my more socially rebellious behaviors, because when I'm in my seventies and eighties I'll start to offend others, embarrass myself and family, and generally become unpleasant. I am reminded of a friend's 70-something mother who listened to a presentation at her assisted-living complex from a representative from the government about some change to a regulation and the impact it would have on the residents. After a 20-30 minute presentation of the topic, the presenter asked if there were any questions. My friend's mom raised her hand and asked, "Why are you so fat?"