I have observed some nice people turn into old crabs when they get old. Other nice people don't change at all.
Combined with observations on the effect of age on energy levels, I have reached a conclusion.
Some nice people are not really nice. They are putting on an act. But the older you get, the harder it is to keep up the act. So as they age, we find out what they are really like, and how much they have been acting all these years.
(This has nothing to do with your topic directly, Suz, but it connects further on down the tracks.)
As for GETTING NICER as you age, I think that mostly comes from learning what to not stress about, and how much a lot of things don't really matter.