I don't remember which talk it was but there was a talk where it was explained that the reason why our trade seems to be dominated by young devs is that it actually is: the number of software developers is doubling every x years and when you have such a fast growth it is simply not possible to not have many young programmers.

I think it was a talk by Uncle Bob where he talked about how many devs there were worldwide when he started programming, how many there were in the early nineties and how many there are today.

The question "Where do old devs go to?" can still be asked (and Ed answered that question) but I think the answer doesn't matter much with regards to the average age of devs during interviews.

A company is going to be interviewing way more younger devs than older ones simply because of the age distribution of devs.

Even if every single (old) dev was still developing software, there would still be very few old devs compared to younger ones. That's why the graph in TFA is shaped like that.

I may be totally mistaken here but that's how I understood that part of the talk and I think it makes sense.