First, I don't know. Pep's biggest impact is & has been in the midfield but has helped players all over the pitch. Piqué is a player that draws some comparisons to Stones and when Pep brought him back from United, he wasn't even playing regularly for you lot. So, you could easily argue there are similarities there in what Guardiola can do with a ball playing central defender who likes to go forward - Piqué even played many matches as a DM for Zaragoza. I can recall many doubted Geri's abilities as a defender. I liked Stones from the first time I saw him under Martinez' first season at Everton but then watched little after so I can't weigh in on what has happened to his development or career from there
For a central defender, yes, age has a big impact. The best central defenders in the modern game have rarely been young players - I suspect the conventional wisdom is that a central defender doesn't really mature into a top flight defender until their late 20s. Experience, especially playing the way a coach wants you to. Not too many people were raving about how great Boateng was prior to Pep's arrival but that changed. I am not sure about Stones because honestly I don't watch him enough to have a strong opinion one way or another. I go only by Pep, if he is convinced then there has to be something there - whether it works out ultimately no one knows but most would agree it's far too early to write his epitaph as a defender