Question

some people in old ages complain of stiff joints .what do you think could be possible reason for it?

Answer 1

The fluid between two synovial joints get dried up, as a result a stiff joint occurs. The fluid is called as synovial fluid.

Answer 2

Arthritis  is of two types in old people -
1)osteoarthritis - progressive wear and tear of joint cartilage
2)rheumatoid arthritis - the body's immune system attacks it's own tissue, including joints.
 
These are some reasons old people have stiff joints. 
Also, synovial fluid decreases with age. This can also be a reason