I just unlocked 少女, which I believe is my third or fourth different way of saying Young Lady, and I’m really confused about the difference between the different readings and when to use certain readings. Can someone explain it a little more for me?
少女 refers to a young female (essentially don’t use this for adults), usually more innocent or pure in nature. One of the synonyms for 少女 is “maiden,” and this carries through. Using this term for an adult usually insinuates that the girl in question is a virgin. For a good example, think of “little girl” characters in fairy tales and such like Little Red Riding Hood.
女の子 is just as the kanji states. It is a child of female sex. This is your umbrella term for all girls who aren’t adults. Some resources may tell you pre-adolescent, but teenage girls definitely still use the term, especially if they want to be more cutesy.
女の人 is what you’d use to point at an woman in particular without any further specification, the bigger umbrella above 女の子. 女 alone is more of the “This is what’s wrong with women!” type of feel whereas 女の人 is more like "Yesterday I bumped into a woman (whose age is undefined/not necessary to mention).
Seeing as you’re not level 47, I assume お嬢さん isn’t one of the terms you’re talking about. Those were the only immediate search results specifically for “young lady” I had, but I’ll cover the related terms I think you may be thinking of.
女子 is very similar to 女の子 in its construction, but has a more specific meaning. While 女の子 is referring to a child of female sex, 女子 is referring to a childish woman. This usually means girls from puberty to around college age. 女子構成（じょしこうせい）is a common term meaning high school girl, especially in the anime/manga subculture.
女性 (じょせい） is a ways beyond your level, but might as well mention it. That would be the term to use for mature, adult women, although the term could technically be used for a mature enough high school female I guess. 男子 and 女子 are what you’ll see on school rosters for students usually.
Risking providing information in short terms, here would be my basic guide:
女 - Female (Woman): Used to reference women-kind or femininity.
女の人 - Female Person: Used to reference a person who is female.
女の子 - Female Child: Used to reference a minor (high school down essentially).
女性 - Woman (Mentally & Physically Mature): College age or higher usually.
女子 - Girl (Physically Maturing, Mentally Immature): College age or lower usually.
少女 - Little Girl (Physically and Mentally Immature): Don’t use for adults unless you’re asking if someone is a virgin (in other words, don’t ).
お嬢さん - Young Lady: This is an honorific title, so it’s suited for Young Lady. Tack 様 on instead of さん and you are in the range of Butler-talk.
I honestly think Young Lady is a terrible translation for anything but the last, unless you know people who regularly call children young lady (I personally have only heard it in shows/movies when a girl gets in trouble 「Get over here, young lady!」kind of thing). Of course, the above list only works if you culturally have the same divide of female, woman, and girl that I do, where female indicates all of the sex, woman indicates age and/or maturity, and girl indicates youth/lack of maturity.
Hope this helps!