Younger people are not smarter. They might learn faster new things. On the other hand, older people understand related new things much better as they already have a large context (experience). The biggest difference of my current self (44) to my younger self is that I did spend much more energy in my projects when I was younger. I created results much faster at the cost of limited consideration. Now, I can still burn…
I also think that the Zuckerberg quote also points to the startup mentality. They want 22 year olds because you can get them to work 100 hour weeks against the promise of an IPO, not because they are "smarter". Hard to convince a 40 year old with two kids to do the same. There has already been a shift to more traditional CS companies (IBM, etc.) as the lack of IPOs has soured some people on the startup dream of being employee #3 at the next Uber of XXX.