Hi, at this age the cause of
knee pain are arthritis,
gout, rheumatoid arthritis, reactive arthritis, bursitis etc. As you don't have history of recent trauma, it is unlikely to be ligament or meniscus tear.
Investigations like MRI, blood tests like ESR, RF, ANA etc are done to find out causes related to autoimmune diseases. Try cold compression, crepe bandage and pain killers like diclofenac etc.