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?
Going to be highly dependent on context. At the cancer hospital? “Old Men” might just be 80+ years. At the office, it might be 60+.
Young adult in a lot of countries will start at 18 or even younger I think? US, adulthood starts at 18 even though a lot of adult things are still closed to them (drinking alcohol, having completed college, etc). So if we mean legally a young adult is probably 18-30 whereas if we mean a young person who is starting adult life we might not mean until 22 or older when they have a chance to start a career, etc.
Elderly and other descriptors might follow the contours of eligibility for government programs like social security.
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”.