The subject comes up from time to time.
My view of it is this:
It really doesn't exist in this industry. Why? Because the median age of truckers is in the mid to high 50's....and in reality, many companies prefer older drivers as they are more steady, set in work habits, more mature, and have fewer home based emotional problems.
What is more important than age, is the persons abilities, and their general health and their ability to pass the DOT physical.
If a driver lives in a company's "hiring area".....then age will probably not be an issue. Maybe if the driver is over 75 or so, but otherwise, not really.