They can refer to themselves as おとこ, although 15 would not be considered an adult man, however old they are, 15, 10, even 5 years old. Example : 俺はおとこです。15歳です。 僕はおとこです。15歳です。 Yes, おとこ is not the first person pronounce. :-)

Depends on the context. Otoko can't be used in place of first-person pronouns such as watashi and boku but it's perfectly acceptable for a 15-year-old boy to say things like "僕は男だから、女子大には行けない (boku wa otoko dakara, joshidai ni wa ikenai)"="Since I'm an otoko(=a guy, male) I can't go to a women's college."

I thing in grammatical approach is right to use OTOKO but in manner of approach impolite if your younger "Anokata" "Anodansai" "(Ano)Otokonokata" "Chunen Otoko no Kata" if close to his age "anohito" "Kare" If older then him " may use "Kare" "Are" Anootoko" word will be charge your position. Might be your Language say That men or That middle age men I think.