My brother developed problems when at school by carrying his satchel on one shoulder. I never really had a problem, despite the weight of the many books I carried around (I once turned up at primary school carrying two bags, containing 32 books in total - all of which I was in the midst of reading.
But I did develop back problems later. This cold have been more associated with my work at the time - stage crewing, so lifting awkward loads and moving from hot venues to cold trucks over again. Also, our family have short legs and long backs, which is an issue. Aaaand, let's face it, I do carry around a bit more weight than is strictly necessary or desirable.
But I haven't really had serious back trouble for a long time (excepting when I drove a lot about ten years ago).
I usually carry a mid-sized rucksack carrying personals (money, keys, phone, cameras (usually two, but small), phone chargers and batteries, SAK, coat/spare clothing, water bottle, flask, medicines...). Not sure how much it weighs, but I usually carry it on one shoulder. Larger rucksacks are carried on both shoulders, but we don't do a lot of backpacking
That said, I can't really ride a bike with anything on my back at all. I think messenger bags look cool, but just are not the solution for me.