Some people say that we are happiest as children, free of adult pressures and responsibilities. In reality, most Americans over 60 are in better emotional health than those under 60. So really, most old men aren't all that grumpy:
Happify
Keep in mind most of the research cited below is from correlational studies that can show a link but not prove causation. For example, if happy people live longer, there are more happy people in the population as it gets older, so the population looks happier as a whole.
There could also be other factors at play and we can't say that being happy about getting old makes you live longer, but these are good findings to keep in mind if you worry about aging.
Knowing that, here are some things that you can do to increase your emotional well being as you age, presented in infographic form by Happify, a web app that helps you build your happiness through fun exercises and games: