How old did a nurse have to be to serve in the World War 1?

the age limits in WW1 varied by country. the United States, for example, required a man to be at least twenty-one to fight, where as in most European nations, countries were so desparate for men that even sixteen year olds could fight. in Russia and Germany, boys even as young as ten years old would fight, and men as old as seventy years old would serve too.