I've had someone tell my (at the time 8 yrs old) daughter that the pack she was carrying was way too big for someone her size. This was based on no fact I could ascertain except the perceived pack size. The pack was filled mostly with down quilts and was well within the percentage of weight someone her age and size should be carrying. Of course my daughter glommed onto that one comment and began to complain and refuse to carry her pack for most of the rest of the trip because it was "too heavy". She had been gloriously oblivious to the (non) pack weight before he spoke up. Thanks random hiker guy. Thanks a lot.
As far as too young to hang? Never too young or too old.