In 1905, Levene was appointed as head of the biochemical laboratory at the Rockefeller Institute of Medical Research. He spent the rest of his career at this institute, and it was there that he identified the components of DNA.
It also takes anywhere from 18 to 24 months to get promoted to O-2 following entry to the Army, so the effective minimum age to join the Rangers for officers is around 23 to 24 years old. Officers can then continue to apply until they are promoted to O-5.
There were soldiers of all ages that fought during the war. The average age for the Union Army was around 25 years old. The minimum age to join the army was 18 years old, however, it's thought that many young boys lied about their age and, by the end of the war, there were thousands of soldiers as young as 15 years old.