If there is no history of trauma or twisting, it seems to be because of ageing process. Knees start degenerating around 45-50 yrs, depending upon many factors like, obesity, overloading, lack of proper exercises, conditioning of Knees.
Where is the Knee/leg pain? Front, sides or back.
How much lower down it goes? Only upper portion or upto ankle?
If it is at back, it can be spinal problem.
Before starting any treatment, it is better to get proper diagnosis.
Next Steps
First step is pain relief by pain killers, local gel, local heating. After that, to avoid recurrence, you should take proper Knee care, do correct Knee Exercises, avoid factors which cause stress on Knees. I have devised programs like Save your Knees, Save your Back to avoid progression of these aging processes.
Health Tips
It it is more than a week or doesn't get adequate relief by pain killers better to show to good Orthopedic Surgeon.