Is reduced mental function a natural part of aging or is it more related to a reduction in physical activity?

I recently decided to take on a small part-time job in a very fast-paced environment. This would require focus, mental sharpness and quick response time. I found myself worried and almost panicky that I might not be able to do this job. I had lived my life in a constant flurry of unorganized activity and as I have improved my organization skills I’ve been able to reduce the need for that fast pace. So am I “out of practice” or is this a normal part of getting old and can it be overcome and improved upon?