Happy Birthday indeed. I get stiff too after sitting and laying down and I'm only 34. I presume your problem is in the hips? ... If so try some Yoga and sleep with a pillow between your legs so your hip doesn't collapse.
D'oh. Now I feel fat. :( (I'm a bit over 12st - and 6 ft tall.)
Kidding.
I think kids like that need to be made aware that their weight can be a health risk to the horse and to themselves. I share a 17hh irish draft mare. She can carry my weight, but with anything smaller I need to be...