Hi
I know a number of tilers who use back supports and they swear by them. Never needed to use one myself.
Please take this next comment in a constructive way .................
If you are fit and heathly and go to the gym - do back and abdominal excercises you will find that your back will improve - your abdominal muscles do a great deal to help support your back, posture and skeletal well being. I am 42 years young now and regular visits to the gym keep me in good stead for tiling.
The only time I have ever struggled with back problems is whwn I had to do a 60 sq metre floor in 900 x 750 x 20 mm thick limestone tiles - by myself. Took 2 visits to the chairopracter to sort my back out afterwards.
Look after your back and knees. However, in able to do this you need firstly to look after your health, any other aids are only secondary