Pretty sure she's a college student. I think one of the earlier chapters mentioned nursing school, so probably in her early 20s. Still an age gap, but not quite that extreme.
Unless I'm forgetting something from earlier chapters that hinted at it, the MC being the reincarnated demon king feels like the author just pulling random shit out of his ass at the last second.:wtf:
I honestly forgot I was even following this one. After reading this chapter and skimming back through some of the previous chapters, I can see why I forgot about it.