I would actually disagree with the replies saying do not bring it up first, but it just shows that everybody is different. It doesn't happen too often, but it's always quite nice when somebody asks me a question about mine. It shows they're interested in it and don't see it as this big thing they can't even acknowledge. I'm not sure about others who stammer but I'll never be the person who mentions it first, mainly because it makes me feel like I'm putting them on the spot and there's no good response that isn't awkward. "Hey, just so you know, I've got a stammer" "...ok" A question from you allows them to talk about it without feeling awkward, and addresses the elephant in the room which in turn will make them more relaxed.