I have had very similar experiences while teaching classes! some 'older' engineers would take longer on a couple of examples but were cruising thru the rest and had better understanding of what the problems were attempting to teach.
Yes, it's certainly all about the cost. But do you really think older engineers cost that much more than younger engineers? Ever hear of 'graduate creep'? I know a young girl who just graduated with a Nursing degree who is making $80K in NYC. I doubt if it's much different for engineering grads...
Older engineers can become a problem for a project when they don't respect the opinion of their younger colleagues. I think, we all should always benefit from the experience of our older colleagues and from the new ideas of the younger ones.
I was once teaching a customer class on the Microwave EDA software we sell and about half the students were young guy, just out of college, the other half were older than 50, like myself. After a leacture session, I would give them a hands-on lab to do. The "young guns" would have it all done in a few minutes, while the older guys would tend to work through each step and try to understand what they were doing at each step. I decided to ask some of the the younger guys about what they had just done, and virtually all of them had no clue, they simply had followed the steps to complete the lab. After that I decided to rearrange the class so that one mentor and one young stud sat next to each other. It turned out to be one of the best classes I've ever taught. The younger guys would help the older guys with the computer work, while the older guys would explain to them at each step what was really going on.
In my recent experience, the ones doing the hiring are the HR people. Often these are 20 somethings that hire based on a list of requirements. The one who most closely matches the list gets the job. Looking for somebody with experience on an Intel 8045678B processor? To them the an Intel 8045678C is NOT the same even if they really are the same. Its like looking for a truck driver with experience driving a Ford, driving a Chevy truck is not the same.... even if it is. Plus all of us gray beards learned how to do CAD on the job and never took College classes in it. Why? Because we learned to do it BEFORE it was offered in College. We learn everything before they teach it in College. But to the people in HR it means.... "no formal training". The people actually in the Engineering departments love us because we have a mountain of skills. The people in HR who do the hiring have no idea that its THEM that don't understand the requirements.
And not to forget the total costs of hiring, with health care ones spiraling out of control. A mix of older and younger is probably the most judicious, with those older ones actively staying abreast of developments.
At least some of the more grotesque aberrations of the fashionable Total Quality Management "gurus" have for the most part fallen by the wayside, where older folks were looked at askance and their experience and wisdom discounted. No, they said, just throw all that out and do various design of experiments stuff and Taguchi this-and-that, with inexperienced people, and you will surely trump all those old fogies. One proud expositor of this looked at me with thinly-veiled contempt mingled with pity when I suggested that actual experience and a grasp of basic principles was still key. He had told us about his training in Japan, which included various team-building "exercises" and the ilk. He didn't invite me to do a fire walk but I could see the glimmer in his eyes.
I suspect he's selling used cars now, or maybe life insurance.
Come on guys. It's not a matter of how old and what they (we) can do. It's a cost thing. It's business. Business is only about money.
A hiring manager will buy the best tradeoff of productivity vs. cost. If entry level fits that bill, entry level is hired at entry level wages. Generally, younger persons demand lower pay so the bias is to hire younger persons. It just that simple.