No premiere date announced yet. The new PV is here. Check out the ANN article for additional information. Synopsis of the light novel adaptation from AniList:
High school student Hojo Saito married his classmate, Sakuramori Akane, the girl who’s his worst enemy in school.
“If you tell the rest of the class we’re married, I’ll kill you”
“I don’t want them to know either. There’s no point in that.”
Their newlywed life wasn’t going so well, and they were constantly at odds. However, as they progress through their daily lives, the distance between them closes, they have more enjoyable moments together, and they gradually come to understand one another. Saito learns about Akane’s cute side, which he never saw before, and Akane gradually began to recognize the feelings that she’s been keeping inside…
The heart-throbbing newlywed life of two people who just can’t seem to be honest with each other begins here!
The laws around age of marriage actually changed recently.
According to this: https://www.moj.go.jp/EN/MINJI/minji07_00218.html
Under a law passed in 2018:
Until April 1st, 2022, people had to be at least 20 years old to marry without parental permission; with parental permission, males could marry from 18, and females could marry as young as 16.
Starting April 1st, 2022, both genders can marry without parental consent at age 18.
So... the answer depends on when the manga was written...
Edit: fixed gender references, grammar and tenses
Anilist says the lighnt novel start on Christmas 2020. And the manga in may 2021.
So they shouldn't have been able to marry since they had to be at least 18 and 16, with their parents' concent.
Yikes
Maybe they're orphans or have consenting parents?