In "Goodnight Sweetheart" We see a close-up of Kousuke's feet as he stops to question Hiyono. He is shown to have stepped off of grass onto dirt, but a few seconds later we see he is in the middle of the street with no grass around.
In the English dub, Kousuke's birthday was given as August 28, 1987 and his age is stated to be 17. Although the anime was originally broadcast in 2002, no chronological year was stated in the series. So, it's possible that the anime was set in 2004-2005, when Kousuke was 17.