Maybe seventeen no ? it feels like they are in their final year now
Setagawa is in fact 15, maybe he turned 16 in the meantime, at the beginnig they are in first year of high school and later the second year starts; it is difficult to get what time of the year it is in which chapter though, Setagawa's birthday is in January and the new school year starts in April in Japan, so I guess he should be 16 by now. Kousuke started at 27 (12 years older) so maybe he is or will turn 28 now :)
I love this manga (and anime) so, so, so much, I buoght all the Japanese manga of this when I was in Japan a few weeks ago and I'm trying my best to read them in Japanese :) But this English scanlations do help me often though... :D Watched the anime three times during the last month and I guess I will rewatch it over and over until they bring on a new season, this would be so great. :3
I love this manga very much and I think it tackles many serious problems that people actually face. My problem comes in with the fact that Setagawa is about fifteen-sixteen and the other guy is in his thirties.