How is the average age at which a worker earns his highest pay changed over the past hundred years?

I would assume the age has increased simply due to longer lifespans. And the consolidation of larger industries post-depression, and post WW11 led to the workforce being able to remain on the job for a much longer duration. Today this is not so true as companies seek to get younger and sleeker. Also, generation X'rs were famous for rapid job change to seek higher compensation. Older workers are forced out because of their higher wages and benefit packages as companies seek to become younger and slimmer. With high unemployment they can get what they want. So over the last century you have a gradual increase until about 1980, and a decrease since.