The big challenge at the moment is coverage; getting enough of the cells successfully edited by the gene therapy to make a meaningful difference. There are promising signs in mice, such as the muscular dystrophy study from earlier this year [1], but in general this is still an ongoing battle, without a standard approach.

The coverage issue is why you can't go out and get any one of a hundred edits [2] and expect things to happen as a result. As soon as that is robustly solved, well, gene therapies will be out there in the medical tourism market, starting with telomerase, myostatin, and follistatin, I'd imagine. Most likely the second of those up front since its already fairly robustly proven in humans via antibody therapies. [3]

I would be extraordinarily surprised to find that there were no humans already tinkering with their myostatin balance via genes or antibodies, given the number of people with access to the technology, and the ease with which such a therapy could be assembled.

[1]: http://www.nature.com/cr/journal/v26/n5/full/cr201628a.html

[2]: https://www.fightaging.org/archives/2016/06/a-short-list-of-...

[3]: http://news.iupui.edu/releases/2015/12/myostatin-warden-musc...