I don't know of a site that lists them. just from looking up several possibilities on baseball-reference stats... I see that A rod and Ted Williams both did it in their age 20 seasons [Ted did it again in his age 21 season]. Pujols did it at age 21. Are they it?
Not saying Justin is a lock to keep up this hitting, as I'm sure he's gonna hit another rough patch before the season ends... but its fun to speculate. he and Mauer have been keeping me real excited in May. I don't know about Mauer hitting .400... but I think Justin has a legit shot at OPSing 1.000 this season.