Well, considering that the only way to tell if someone is a Cylon or not (if you don't already know what all the models look like) is a very time-consuming test involving breaking down bits of their tissue and studying it for artificial compounds, and considering that humans and Cylons can interbreed naturally (if not frequently), and one of the main plot-lines for the whole series involved a Cylon who fell in love with, had a kid with, and married a human ... the only part that's implausible in the story, to me, is that John (the psycho-Cylon) managed to figure out how to make Daniel physically younger. That he then grew and aged after that had happened is perfectly consistent with what we've seen of Cylons in the show.

(Oh. There is one other way to tell someone's a Cylon--their spine glows red during sex. But unless Daniel's ever had someone behind him while he's having sex, which is a lot less likely for a straight man than a woman, he wouldn't know that.)