Board Thread:Questions and Answers/@comment-43717534-20200226190021/@comment-27869316-20200227125228

Since most high schoolers in Japan turn 17 during their second year of high school (as far as I can tell) it's likely when his age was given they're counting his age by the end of the year as a whole, rather than accounting for his age in accordance with each episode/month in the anime.

Hope that makes sense/helps!

-Fox 12:52, February 27, 2020 (UTC)