I made a kid's bow outa a hickory board, came in at 32#, 48" nock to nock, had a set of 23 inch poplar arrows that were 5/16th shafts.  The kid that got it shot a lot of cottontails with it and his father glued on a broadhead and shot a small doe at 10 yds.  Broke a rib going in, didn't get pass-through, but the deer ran over to the tree row behind the barn and died in a few minutes. 
I do not and will not recommend ANYONE AT ANYTIME go lower than a 40# bow for hunting deer. That idiot got lucky.