Board Thread:Speculation House/@comment-4830106-20140921022402/@comment-25555436-20141118232423

He once fought a 1000-years old Ursa on the top of a mountain, without his Blunderaxe, while surrounded by thousands of almost-equally old Grimm of all kinds, that attacked him all at once.

And that is why only 2 Adult Grimms have been shown in the show so far. It's also the reason for why the Deathstalker was hiding in a cave and why the Nevermore was so anormaly big (from exposure to Port's Portness).