Factual error: The word "intel", used several times, is first recorded in 1961. It certainly wasn't in use in 1918.
Factual error: When Steve gets dressed after bathing in the pool he zips up his fly. The film is set during the First World War. The zip was not used on clothing until 1925 and did not become common on flies until long after that.
Answer: It would be more accurate to say they grow to a certain age then stop aging. Similar to the elves in Tolkien's Lord of the Rings.
jimba