Tom Stewart calls my attention to this article in The Washington Post about Hal Holbrook, who is still (amazingly) touring with his one-man show, Mark Twain Tonight. I didn't realize it but Holbrook started his Twain act in 1954 when he was a 29 year old actor playing a 70-year-old man. Now he's older than the Twain of his act and almost the age Twain was when he died.
I've never seen Holbrook do Twain in person. I saw the 1967 TV version (which you can order on DVD) but that was long ago and only 90 minutes. Holbrook has hours and hours of Twain material and reportedly varies the show every performance for his and the audience's amusement.
I wanted to post a link to a site that listed where he'd be performing it in the future but I can't find one. If you can or if you hear about him heading out west with the show, let me know.