CKD moving at fast pace

I only found out I had Stage 3A CKD in August. My blood records,when I managed to obtain a copy was headed 'no further action'. I had been taking Ibuprofen for painful knee arthritis for years but I wasn't advised they could affect my kidneys. I complained to my doctor about not being informed and he told me kidneys decrease with age and as my creatinine level was ok it was nothing to worry about. He said I would have a blood test every four months to monitor the situation. I had a blood test last week which now shows my eGFR is 42. That's a drop of 25 % which is very worrying. If it increases at this rate my kidneys will fail this time next year. I don't have diabetes or high blood pressure. I'm fit for my age and play tennis four times a week. I'm 71 years old.