Most Helpful Girl
It probably has to do with the way you dress/act/talk
possibly has to do with your physical looks and maybe the tone of your voice.
someone who dresses close to "skater" and "emo" or "scene" styles would be more classified as a kid in my mind. not knocking those fashion genres (or ways of life or whatever) because I find them very attractive. but also, the "grown up" side of my brain does see it all as more of a "kid" classification.
verses classic t-shirt and comfortable jeans kind of guy would be seen as more as a... well... guy.
If that "guy" has strong leadership qualities, shows mature confidence, and has a noticeably mature body, then I'd think of him as a "man" most likely.
So there are probably very few 21 year olds who would come off as a man in my oppinion. I dunno. I kinda went into a lot more detail in this answer than I planned ha ha hope that helps