When in person I just don't bother, they always ask me in the end and I tell them. I just say, "I have a speech impediment" or "I have a stutter". Then about half of the time they ask more questions and I answer them. "Yes I've always had a stutter" "No I don't know the cause of stuttering, there is no true defined cause of it as there are multiple theories that try and suggest a cause for it and by doing so disproves other theories that don't directly support it." That kinda thing. I don't bother telling them unless they ask about it.