Does a stutter usually get better with age? And if yes, why/how?

Does a stutter usually get better with age? And if yes, why/how? Does it? My uncle had a severe stutter ever since he basically could talk, and it was severe until in his twenties I believe, he’s 44 now and it’s now mild for quite a long time. You still notice it sometimes, though, but a lot less than before. I have heard lots of people say it generally gets better with age. Is that true? And if yes, why?