Let's take an example case where two engineers are working in a second tier city earning $85,000 per year in salary. One is 25 years old, one is 53 years old. The company covers the entire cost of the employee health insurance. The health insurance for the 53 year old will cost a minimum of 2x what it does for the 25 year old. The cost of health insurance begins to aggressively increase after roughly 38-40. By the time the 53 year old employee reaches 60-63 years old, the insurance cost is now 3x that of the 25 year old.
The 53 year old will solidly cost an extra $6,000 to $10,000 in health insurance on top of that $85,000 salary, versus the 25 year old. The 60 year old might cost $12,000 to $18,000 more in health insurance.
All of these figures are minimums, the numbers get much worse if you have a nice health plan.
If you're in a second tier city paying your engineers $85k, $15,000 per year in extra cost buys a ridiculous amount of training to improve the 20 something engineers.
If you have 20 engineers, over ten years the cost of employing all 55-63 year olds, versus all 20-30 year olds, could easily be a minimum extra of $3 million in just health insurance costs (with a total salary over ten years of $17 million). The more likely cost is far higher.