Killen is a new one. According to Interglot, it means lad, chap, dude, etc.
Pojken and killen have the same meaning, but "pojken" is an older word than "killen". (but "pojken" is still commonly used) Pojken is used for kids but killen can be used for all ages.
Tack for the clarification. I got this one wrong the first time due to not knowing "killen". Yet another curveball in true Duolingo fashion.