How old is the magikaverse characters and also is it too early to ask who the characters going to be in magikaverse? :D
Hiii, thank u for ur question! When the story starts, the characters are aged around 17-19 years old depending on the individual. Some of them will start out in the last year of highschool, others will already be out, and as the story progresses, they all go through either a college/apprenticeship system or their first jobs (all the while fighting evil as magical girls on the side, huehue).
I think most magical girl stories feature characters in middle school age, at least the most famous ones, but I thought it would be interesting and refreshing to have a magical girl story that is about the first steps of adulthood: higher education, getting a job, discovering your place in the world, that kinda deal. (Again, all the while with that magical girl side gig. Life ain't easy-)
Also, for the beginning of the story, I thought it would be a fun concept if Ink, still pondering what to do with her life since she's got no clue yet, is granted the power to transform into a magical girl on her 18th birthday. Yippeee, what a gift!
On the characters that will definitely be featured and play a bigger role, we've got: Ink, Geno/Error, Dream, Blue, Nightmare, Dust, Red, Frisk, Chara, Flowey, Gaster, Reaper and Cross. (Actually also one more, but it would take away the surprise I've planned, ah pssssht. Feel free to speculate, however~)
Other au skeletons or Undertale characters in general might appear as background characters, but not be too relevant to the story. Depending on my motivation levels and how much demand there is, a spin-off series with different main characters is possible, however. (Like... I really want to design a magical girl outfit for Outer... galaxy theme...)
If there are any more questions, I'll gladly answer them!