Headscratchers / Fly Me to the Moon

• In Episode 3, Nasa is 18, while Kaname is 15. Despite that, they're apparently only two years apart in school, since Kaname was Nasa's kohai in middle school (which means at the time of the accident, he was a third-year while she was a first-year). Is Nasa old for his grade (i.e. birthday near the start of the school year) while Kaname is young for hers (birthday near the end of the school year), or is there some other explanation for this?
  • Didn't he lose a year because of the accident?
  • Nasa's birthday is listed on the marriage registration as May 5th
  • Nasa isn't in school. He skipped high school and started working, both to try to find Tsukasa and to be able to support their married life.