Board Thread:General Discussion/@comment-31071571-20180108195146/@comment-29759785-20180108212319

Timline shows us pretty clear the answer for that:

Kazuto was born on 7th of October 2008. SAO death game starts on 6th of November 2022, so that makes Kirito 14 years old at the beginning of Aincrad arc. The game is clreared on 7th of November 2024, so Kirito is 16 years old at the end of Aincrad arc.

Hope it helps ;)