[porlw]

1. Computer tech seems to move in a ~20 year cycle, so after that amount of time you will have "seen it all" to a certain extent, and you can apply your early experience to the new wave of technologies. For example, centralized mainframes -> personal desktops -> centralized web servers -> personal portable devices.

2. At 20 I was an arrogant know-it-all, 30 I had solid experience in certain things, by 40 the experience broadens and you find you can take a higher level view of things.

3. A 30yo offers knowledge of a certain specific technology (what to build). A 40yo offers experience (especially what NOT to build), which makes them more valuable.