It's an interesting question.
My great-uncle was born in late 1884 and emigrated to Canada at age 20.
He was therefore coming up to 30 when he enlisted in the Royal Canadian Engineers six weeks after the declaration of war. He gave his younger brother's date of birth: late 1886.
Why would he do that? Could it be that promotion to officer was open only to those under 30, or was there some other reason for fibbing?