Obviously teenager is 13-19.
“Young adult” would start at 20, but where’s the cutoff at the upper end? Similarly, what’s the range for “adult”, “old”, “elderly”, " ancient"?
If someone asks for responses from “old men”, how do I know if it applies to me?
To add the the context dependence: “Young Adult (YA)” books and media are generally geared toward teenagers. At the library, YA is reading materials that are too complicated to call “children’s books” but still a lot easier to read than general fiction/literature/etc. From an age standpoint, kids often start reading YA stuff in late elementary school. So… at the library, young adult is close to synonymous with “teen”.