I'm a grad student in the aging bio field. Steve Horvath is well known and well regarded, but I'm pretty sure he just did the regressions. Other characters on the paper are more unusual including a former Investment Banker from India. I think the mainstream aging community view would be "wow these results too good to be true and I wish they presented more methodical details and individual data points". If it's true, it's quite exciting. In my own work, I've seen the rejuvenating effects of young blood.