Any actor who could play simple, appealing, youthful roles was a juvenile. The juvenile lead was the male version of the ingénue. Child performers who grew a bit long in the tooth might become juveniles and many actors continued in juvenile roles even as they aged. Before child labor laws put an end to the practice, there were touring companies in which all roles were performed by children. The appeal was largely novelty, but economics must also have been a management incentive. For example, in 1885, the very year of the premiere of Gilbert and Sullivan's The Mikado, Alice Nielsen toured Missouri as Nanki Poo in a juvenile Mikado company.
The Historical Dictionary of the American Theater. James Fisher.