This still happens to me quite frequently (28F) and honestly, if you feel comfortable, if feels really good to respond to this with "Oh, I have stutter actually". Very matter-of-factly. I often add "Sorry" in there, but I am trying to move away from this because I don't need to be apologizing. It's great awareness for them with the hope that maybe they won't be a-holes about it next time, and also it feels kinda good to watch them squirm and apologize. It does take time to feel comfortable saying this, but I've found it's the best response for my mental health and their awareness!