Ask HN: Why do “Young” programmers think they know more than “Old” programmers

I'm not accusing every "Young" programmer of this but often on HN I see people post and make this assumption. One guy looking for work was told that he still had a chance because he was "less than 35".

I've also heard people say things like "I've got nothing against older programmers but...."

That was from a guy with only 1 programming language under his belt and experience of only 1 software process.

So my question is, at what point does a younger programmer become more experienced than his/her older more experienced self?

When do they know more languages? Experience more processes and organizations large and small? Have more side projects? Have dealt with more difficult people? Have read more books on CS theory? Have learned more frameworks and watch frameworks come and go?

Just curious.