Most folks I talk to about juvenile series have heard of Nancy Drew, the spunky girl detective who travels the world solving mysteries with George and Bess. I've only read the classic stories, but I enjoyed those.
Fewer people have heard of Judy Bolton, also a classic girl detective, but I enjoyed her adventures more. Nancy Drew has been called the perennial teenager, but Judy Bolton grows older in each story. Judy even marries an FBI agent.
The Judy Bolton series ran for thirty-eight titles, during much the same time frame as the original Nancy Drew series. Unlike the Nancy Drew stories, however, one author, Margaret Sutton, wrote the Judy Bolton adventures under her own name. The series might have continued past thirty-eight, but the publishers killed it because Sutton wanted Judy to have a baby. The publishers thought it would make Judy seem too old.
Few classic juvenile detective series had the main character grow up, and I don't know of a longer running one than Judy Bolton. I doubt there was a better.