Yes, kille used to be a slang word but is getting more and more common so it's just an everyday word now. People still say pojke a lot too, but it tends to be used more about children. So I'd say it isn't old fashioned to say pojke about a 4-year-old, but it definitely is if you're talking about an 18-year-old.
The words tjej vs flicka work the same.
Also, en tjej and en kille can be 'a girlfriend' and 'a boyfriend', but en flicka and en pojke cannot.