I'm 73 and haven't noticed any decline in my cycling fitness. However, I didn't start cycling until I was 67, so I'm not comparing my fitness now with my fitness when I was in my 30s or 40s. I was a runner for more than half a century, but had to quit because of knee problems. Maybe I was more fit when I was running, but I feel just as fit and active now. So, to your question, you should have many years ahead before you feel like your fitness is declining. BTW, I'm not monitoring my performance, so it may be that I'm not as fast or as strong as I used to be. I'm telling you how I feel.