@Steve01 No...I didn't see their year of birth anywhere. It just my deduction since in some chapter, it explains that Yotsuba got held back and the quints have to transfer out of her old school.
Now you mention it, I didn't know whether they continue the year or repeat a year when they transfer to their new school. I always got an image that they still have to repeat a year when they transfer to the new school, made them 1 year older than Fuutarou.
So Maruo, it seems took on the quintuplets because Dena was very dear to him? I had thought they were together, but to take on someone else's kids without being in a relationship, damn maruo is something else.
Yotsuba never got held back a year. She got expelled in her current year but resolved her problem by transferring her to another school that same year. And her sisters followed her.
If she got held back a year, then she wouldn't be in the same year and class as her sisters right?