This question rests on two premises that are both questionable. First, that the early space program's engineers and functionaries were mostly young, and second that this was due to a deliberate exclusion of older engineers, controllers, etc. from the Apollo program.
To address the second point first, I would submit that in large part, no such pool of older experienced personnel existed, so any bias towards younger personnel was merely because hiring processes tend to skew younger when relevant experience is not available. That is to say, if nobody has experience in the field, you might as well hire younger people rather than people with unrelated (or only marginally related) experience. Sure, the United States had Paperclipped a bunch of previous-generation German rocket guys, but from that point forward the space program was pretty much breaking new ground with every project. There was no existing standard for how to run a launch or how to build a good rocket. Mercury, Gemini, and Apollo were pushing into unknown territory and writing the rules as they went along (and largely shared the same group of people, for whatever that's worth). Yes, the space program included a lot of younger people, but that's not because older experts were intentionally excluded, there simply were no existing experts that they could have drawn on in many of the relevant fields. (In fields where experience was available, such as project management, they did indeed hire older, more experienced people.)
But that said, addressing the first premise, I think the question potentially ignores a lot of older engineers who were working with NASA simply because they weren't technically working for NASA. For example, John D. Clark (author of the excellent book Ignition!) was in his mid-50s by the time the Apollo Program was even starting, but his work on fuel chemistry was done under the auspices of the US Army's Liquid Rocket Propulsion Laboratory, not NASA. The Saturn V's massive F-1 engines were developed at Rocketdyne, a division of North American Aviation, and the metallurgy research involved was even further removed from NASA itself. Saying "NASA hired young engineers" may be entirely factual while also being completely deceptive, because much of the work on the space program was done by contractor companies, and thus NASA didn't hire any of those engineers, regardless of their ages.
