I remember Matt Lamb making a ton for the 2's when he was about 14/15. Can't remember anyone that young doing it since until now. I could very well be wrong.

Sam Hain must have done, and Ian Bell.

I can't find on Cricket Archive that Matt Lamb, Ian Bell or Sam Hain scored a hundred for the second team or even came close at age 15.
I thought about Dennis Amiss but I don't think he did either. He batted down the order in his early games.
Maybe there are others who managed it but I can't think of too many more likely names.