In reference to the IzaMika ship and it being inappropriate due to Mikado's age. I would merely like to point out that Durarara!! has a light novel series that goes along with it. And the characters do in-fact age. The novels go from 1-13 (not counting SH), the anime ends around Volume 3. Mikado is 15, Izaya is 23. By volume 13 of the novels, Izaya is 27 and Mikado is 18. This is where a majority of the ship comes from. Mikado is a legal adult in the novel canon world where most of us are.
Oh yeah, I mentioned that in my original post, but I really thought it was worth contesting the whole foundation of the argument in the first place. And I have seen fanfiction and fanart that don’t depict Mikado at age 18, so that defense isn’t universal, unfortunately. Thanks for stressing it, though!