The more younger you are the better it helps. Because once you've grown up your brain is wired to stuttering habits and anxiety. So the best time is when you are young because you are still growing and you can adapt to those more easily. But that doesn't mean that doing speech therapy as an adult is worthless. I went to speech therapy a couple of times but it didn't help like I was expecting. But I made my own little methods to help me talk more easily. So I didn't use full on speech therapy methods for every word. At the end of the day the goal is to say whatever you want without holding you back.