I'd risk to say, at whatever age you choose to. On this very forum I have seen comments from 60+'s who were snowed under work and very happy (and not with 'old' tech), and also I have seen early 30's who were worried about them becoming too old for the industry. I believe it depends on your motivation, and maybe how you came into industry. Some are self-taught enthusiasts and some are formally trained workers, and some are in-between. Some are burned out by their circumstances, some are not. Some have family to feed and some are living life of a single. Superposition of all of this will result in mindset and how the one feels about their age related with what they do for living.

I'm in my early 40's and I think I will be doing what I do now until I retire (in whatever form it will be done, and if I'm not hit with brain desiese).