I think his point is more related to the learning curve.
If you're in your fifties it doesn't mean that you are stupid but it does mean that you're going to have to learn a lot of technologies from the recruiting company's stack.
Now if they can find someone that already know's them, their return on investment will be faster and safer.
The older developer will also most likely not program on his free time for family reasons or because he has more interesting hobbies.
Now this is controversial but I still would say that this is career / life choice which means that not having a very sexy resume won't get you everywhere no matter what your experience is.
This of course sounds very "hipsterish" but if you loot at the kinds of companies that are talked about here (start-ups and ex-startups that grew), it does make financial sense.
I'm not a psychologist but I would also wager that age gap is lowering team performance due to a lesser bond / closeness between the members.