How long will you continue cycling?

I plan to try and maintain a good level of fitness indefinitely. Cycling is just one of many actvities i love that i'll hopefully continue till something forces me to stop. I love the focus cycling, running, climbing can give you.

Of the three activities you cite, cycling is the one most likely to last into old age. Running typically puts significantly more stress and impact on the knees and ankles, for example (although people riding too high a gear on fixed can cause themselves some serious knee pain). Even where people develop mobility issues, they can often use trikes.