@PowerRiderUltimate64 Again, Kirito is 19 by the time he and Eugeo are taken to the central cathedral. But ONLY mentally (or rather, his soul is) this is because he does not physically age while in the STL, but due to the Fluctlight Acceleration (FLA) rate, his soul ages at a faster rate.
By the time they eject Alice from the Light Cube Cluster and defeat Vecta/Gabriel/Subtilizer, only 2 months have passed in the Underworld.
As for what is currently happening in the Light Novels, and I repeat, in the Light Novels (because the Manga is NOT the source material, and is simply another adaptation) between the events of Vol. 21, 23, & 24 Kirito has STILL not had his 18th birthday. So no, I do not agree that Kirito "might" have turned 18 "because time flies by fast in SAO".
TL;DR: No, Kirito is not 18 in the current events of the source material. And also, the source material is the Light Novels, not Manga.
P.S. I can say both of this things knowing them to be fact because I take the time out to do my research and read the books, which I advise you do as well if you can. The LN's are definitely worth the time, and are explain things a lot better than the Anime.