
Looks to me like the author simply made a mistake here and meant to write 15 years or something like that (or maybe after 5 years of marriage). We see a drawing of him when it happened for the first time and he clearly isn't five years old in that panel (it also looks like she didn't really age between the first panel she's in and the one in which she gives him the money).
Not that this makes it any better. That's still a horrible thing to do.

I actually did consider that maybe Roku -san made a typo or something and it was 15 years old. Well, the second panel where he received money from his step mom was a fast forward scene, i think? (It was him being jaded and had lost all the fucks to give over time)
But yeah, it's not like him being 15 years old made it any better.

Yes I mean the author. There's also a five in the raws (not that I can identify anything else ( ̄∇ ̄") ).
And, yes the panel where he gets the money is a fast forward scene but that's my point. We see both the mum and him when it first happens (the panel before he gets the money, that's him when she first came to him) and then when he gets the money his mum still looks the same as she did when she first rapes him. But he should be at least 17 or 18 in that panel so 12-13 years later.
We NEED TO PUT HER IN AN ELECTRIC CHAIR!!