I've noticed the same. I think self-confidence grows with age and we become more and more comfortable in our own skin. But then at some point (perhaps in our fifties) we pass a threshold and enter the witty curmudgeon territory.

Edit: on the science bit. Yes, this is merely what I tell myself. But if we were to test this, perhaps we can do so by monitoring the glucose consumption of brains of introverts in different types of social situations?