We only have two episodes of Twenty-Five Twenty-One but I think if you like this show then you'd really enjoy When We Were Young.

When We Were Young is a Chinese show about students in the year 1996, so there is a similar vibe going on between these two shows. Although When We Were Young doesn't have an age gap between the two leads, this show does well to really create the curiosity of "Will they get in the future?" from the viewers (like we get from 2521).

It's very wholesome and full of fun family and friendship relationships. Even though we don't know much about how the rest of 2521 will play out, I think When We Were Young is a good show to watch after/when watching 2521.