At some times and in some places sixteen was considered the age of adulthood for a woman. Although the phrase "sweet sixteen" is polite and not considered rude or vulgar, the "sweet" does carry a faint idea of sexuality, as in "sweetheart" and "sweetie." It is part of the longer catchphrase "sweet sixteen and never been kissed." In the United States, a girl's sixteenth birthday party is likely to be called her "sweet sixteen party." If a girl says her birthday is coming, and someone asks "Which one," and she says "Sixteen," people are certain to say "Oh, sweet sixteen!" or "Sweet sixteen and never been kissed!" I don't know the history of the phrase, but it has a long one. The song "When You Were Sweet Sixteen" was a hit in 1898. I don't think the song is the origin of the phrase. Wikipedia reminds me how the song begins: I love you as I never lov'd before, Since first I met you on the village green. Come to me, or my dream of love is o'er. I love you as I lov'd you When you were sweet, when you were sweet sixteen.

"Sweet sixteen" is a party that signifies coming into adulthood for a young girl. There is something similar in Spanish called a Quinceañera. Except for 15 years old rather then 16. Don't be confused by "sweet" in this context. The two words go together have a specific meaning.

Well, the saying is widespread because it is used in a famous song. https://www.youtube.com/watch?v=WusQjMnD0cs "Tonight's the night I've waited for Because you're not a baby anymore You've turned into the prettiest girl I've ever seen Happy birthday, sweet sixteen What happened to that funny face? My little tomboy now wears satin and lace I can't believe my eyes You're just a teenage dream Happy birthday, sweet sixteen" 'Sweet' means nice, and also innocent. At the time the song was out I think most people thought this was ok...but now we might be troubled to think of an older man singing this about a 16 year old!! Mostly, though we still say 'sweet sixteen' because we like the sound of 'sweet' before the word 'sixteen'. It sounds poetic, I guess.

And in the Internet Archive, Google found me a 1919 piece from "St. Mary's Muse," apparently the school magazine of a women's preparatory school (private high school) which gives some more insight into the cultural context. (Notice that the author, being class of '23, would apparently have only been 15 or 16 years old herself). https://archive.org/stream/stmarysmuse19191920sain/stmarysmuse19191920sain_djvu.txt Sweet Sixteeo Mabel Norfleet, '23 At exactly 7:05 A.M. on Friday, February the Thirteenth, Jacqueline Pennington awoke to the realization of the fact that she was sixteen. She was thrilled, yes, that is the word, thrilled! She had always, from her very childhood, thought of sixteen as the age; in fact, when one was sixteen one must be quite a young lady, well versed in the ways of the world. "Sweet sixteen, and never been kissed," she repeated to herself, as she lay warmly in bed. True to life. She knew nothing of the world, of men, etc. She would know, she would begin that very day! "My goodness, I have been a kid — I don't believe there is another girl my age who has such childish ways. I'll change them though, sure as I was a tomboy and a good little sport, like Jim says, yester- day ! Why, I even wear my hair down my back. Guess I'd better get up and begin my transformation. It will take some time to get my hair up on my head." So, suiting the action to her words, she jumped out of bed and began her first "sweet sixteen" toilette.

I decided to explore how old the longer phrase "sweet sixteen and never been kissed" is by doing a Google Books search. The oldest is 1907, and there are about fourteen relevant hits between 1907 and 1930. Interestingly to me, some of them refer to _MEN_. Here are two of them: 1) From the magazine, the Saturday Evening Post, '"Sweet sixteen and never been kissed. Ain't he the cunning little thing, ladies?" Well, that was a goat getter for fair. The kid's face got red clear up into the hair and his ears looked like a couple of rose leaves.' It was used to tease a young man, making him blush! 2) Another: "Johnny looks like Sweet Sixteen and never been kissed, while Fred appears as a man expecting a large inheritance from a wealthy maiden aunt. We can't all be handsome." A few of the rest: 1926: '"Sweet sixteen and never been kissed." Only that wasn't true. She had been kissed — lots of times. You went to parties and between dances you sat on porches, on the railings, usually.' 1922: from a company newsletter, for employees of the American Woolen Company, next to a yearbook-like picture: "Eleanor Is Some Girl. Sweet sixteen and never been kissed. That's pretty Eleanor Harpring. Besides being a good reeler, and a heart-breaker, Eleanor is an excellent dancer. Eleanor is a prime favorite with all who know her..." So, the phrase at that time informal, polite, considered a compliment, slightly teasing, and with slight overtones of sexuality. "Sketches Drawn from Marshall and Vicinity, Past and Present," suggests that "the old phrase 'sweet sixteen and never been kissed' originated in connection with corn shucking parties. (Corn means maize, and chucking means removing the tough green leaves from the ear). "There were red ears of corn and white ears of corn." That is, the kernels were red or white, and you couldn't tell which until the ear was shucked. "If a girl shucked a red ear, her sweetheart could kiss her." And that, unfortunately, is all Google shows us.