Martin writes:

OK I dug around and Alfred Binet’s intelligence, or IQ, test used on enlisted
men also included an assessment of mental age. I guess it should have been "The
average mental age of US enlisted men tested during WW1 was that of a 12 year
old".

That doesn't make a lot of sense either but ...

It doesn't, and modern tests are usually normed differently, although the
concept of "mental age" is still used sometimes for scoring.

In reality, a 25-year-old is no more intelligent than a 12-year-old. The only
differences are in experience and acquired knowledge, although both obviously
have quite an influence on overall competency in life.