When I searched for "Killar and tjejer" in google images (it's good to get visual context for words like these, since they could refer to a different age range in each language) most of the images were a bit sexually suggestive, and of teens or young adults. Can these words be used for little boys and girls as well (which is what the English usually refers to)? It seem like it's more like "guys and girls" (maybe 15-30 year olds), while boys is in my opinion more like 0-15.
Also is there any reason the images for "killar och tjejer" are more sexual or romantic, when you don't really get that for "boys and girls" or "guys and girls"?
Killar och tjejer can be basically any age. People say that about their newborns: Det blev en kille/tjej! and there are always some older people who describe themselves as kille/tjej, 70 år or things like that (although others will snigger, it's not that rare). And in the sense boyfriend/girlfriend, it's even less strange. Farmors nya kille 'Grandmother's new boyfriend' – well, it's not that easy to find a better word for it.
Still, the perfect idea of kille/tjej is a teenager or young adult.
On the other hand, your grandmother may refer to young men and women as pojkar och flickor, but that is more rare than the other way around. It sounds distinctly old-fashioned. I never hear teenagers refer to their friends as pojkar och flickor. (Maybe in Finland, I don't know for sure).
So your google findings make perfect sense to me.
PS I forgot to point this out: don't forget that min kille/tjej is normally 'my boyfriend/girlfriend', but min pojke/flicka if it's even said at all, is 'my son/daughter'.
This is a question about pronunciation: with words that have tj, (such as tjugo and tjej) I have heard the tj be pronounced like a kind of throat scoff kind of sound. Same with the "sk" in männiSKor. Can you pronounce them the throat way AND the SH way? Because I have heard natives pronounce them both ways.