A disability is an impairment that significantly impacts a major life activity. The ADA includes “speaking” and “communication” as a major life activities. So, if the stutter significantly affects one’s ability to communicate, it will be a disability. It's not likely to get you disability benefits in place of working or something, but it should definitely be protected from discrimination and get accommodation like other disabilities. And by accommodation I mean like, people can listen 20% longer in a business presentation for a stutterer to say the same amount of words, especially since stutterers often value brevity more than normal.