From my experience, the "common assumption" that your brain works best only when you are in your 20s or 30s is a complete and utter nonsense. You might get a bit more burned out with time or simply become less passionate about technology in itself or professional careers in general and instead care more about your real-world life, but if you keep your body in good physical condition and keep pushing your brain, you only become more efficient and faster at programming every year, due to your accumulated experience and improved intuition. (Consider the Linus Torvalds guy.) It is only the corporate environment that actually forces you to switch to managerial responsibilities.