"Young at Heart" is another episode I don't remember well. The salamander hand was memorable, but I had no clue what else would go on in the episode. After watching the episode, there's a good reason why. It's boring as hell.
The idea of a doctor using genetic experiments to de-age people and the government wanting his work is interesting, but most of the episode is spent on procedural elements that go by exceedingly slow. The X-Files rarely works when Mulder and Scully track down leads like on a conventional procedural and when they do, the pacing isn't nearly fast enough to keep me interested.
The episode is also a scattershot of information. Mulder's former supervisor, Reggie Purdue, shows up at the beginning of the episode and is quickly killed. The monster in question, John Barnett, is from Mulder's past, but that thread is soon forgotten. The episode is scarier than "Lazurus" and has more uncertainty which is a plus, but overall is a weaker episode.
Score: 8.0/10