Tbh ive seen this trend alot. My father, uncle and 2 brothers had very severe stammer as well. They all recovered around the age of 13-16. Like completely recovered. You can barely notice they stutter if u really pay close attention to it. My younger brother is turning 15 next year and his stutter has improved so much this year. He's talking to his buddies in online games for hours. I was just the bad fruit in between. Turning 22 next month with stammer at its worst. I often have to take my younger brother to market or make him pick up muy calls now. Stopped talking to friends unless its necessary in class. I'm happy for him but i guess it makes me kind of an asshole that I'm extremely jealous of him as well.