There are a variety of reasons why some older people may

Become grumpy and irritable, including:

 

• Natural changes 

  As people age, they may experience diminished impulse control, become more sharp-tongued, or say what's on their mind. 
• Other factors 

  These can include lifestyle shifts, age-related challenges, isolation, loneliness, Alzheimer's and dementia, losing a job, retiring with little or no savings, constant pain, being disabled, or rarely getting visitors. 
• Social rules 

  Younger people may be motivated to be nice and present themselves positively to gain people's favor, but that may not be as much of a motivator as people age. As a result, outward expressions of grumpiness may just be a sign that someone is unconcerned with social niceties.