People are focusing too much on "pedo vibes" to get the real vibe this manga is trying to convey: A man stuck in the worst midlife crisis imaginable, with a depressed daughter and a dead wife.
You think hooking up with an older woman would help? Look at how the older woman at the school treats him! Constantly judging him as a failure of a father despite doing his best.
What he needed was someone young enough to not care about any of society's norms, and to just want him to be happy.
It's not the age gap that's the focus here, but what it represents for the mentally unstable main character; salvation.