Your question presumes that 21st cenury standards were applied in the 19th century. There was no certification of nurses until late in the 19th century. Florence Nightingale, “the mother of Nursing” didn;t do her work in the Crimea until 1954. The first schools opened in 1873. Before that there was an ocasional handbook. But there was no standerd “minimum age”. If you are writing a story, nursing in 1890 was vastlly different from nursing in 1810. But both were in the 19th century.