For me, the cutoff has nothing to do with body weight. I weigh about 165lbs, work out regularly, and am strong for my body weight. Lifting and carrying 50lbs around all day is no big deal for me, strengthwise.

However, any pack over 30lbs or so (rough estimate, I've never weighed my pack) starts to get uncomfortable after a while, as the skin and flesh on my shoulders and hips grow sore from the load. On an extended trip my joints grow gradually more immune to the bruising, but that first week can be gruelling.