For whatever reason, it seems the AL has been a little better at developing pitchers who meet our 50-win criteria. Whereas several National League franchises had to back to the early '80s or even the '70s to find their third guy, only two AL teams had to go beyond the late '80s -- the Tigers and Red Sox.
A quick note on the Red Sox and Yankees. This chart shows how those two teams have relied extensively on trades and/or free agent signings to fill out their rotations. After Jon Lester, the next Boston pitcher on the list is Roger Clemens, who debuted in 1984. Since then, most of their top pitchers have been acquired via trade (Pedro Martinez, Curt Schilling, Josh Beckett, Derek Lowe), or in the case of Tim Wakefield, after the Pirates released him. Clay Buchholz, with 30 wins, is their current homegrown guy with the best shot at winning 50.
For the Yankees, we get Chien-Ming Wang, but then we have to go back to 1995-1996, when Andy Pettitte, Mariano Rivera and Ramiro Mendoza debuted. Rivera and Mendoza were primarily relievers, of course, so if you want a third starter, you have to go all the way to Ron Guidry. (I didn't include Orlando Hernandez, who was really an international free agent and not a homegrown product.)