Now here's the key claim: "increased dramatically for engineers during their 30s but that these increases slowed after the age of 40. At greater ages still, salaries started dropping, dependent on the level of education. After 50, the mean salary of engineers was lower—by 17% for those with bachelors degrees, and by 14% for those with masters degrees and PhDs—than the salary of those younger than 50. "
Two points:
1. Would we expect a linear increase in salary with age? I would not. At some point the salary would likely 'top out' in any job.
2. The part about over 50s ignores a very important fact from the study: the over 50s work less (and therefore earn less). To quote the book: "Workers over the age of 50 are much more likely to work less than a full year [...]. One in six engineers aged 51 to 65 reported being paid for less than a full year of work in 2005 [...]". The book goes on to make a confusing claim about whether that's voluntary or not (jumping to the 2002 downturn and talking about interviews with engineers but with no data).