Err...The Census has been around for well as long as record keepers have. During that time, the Victorian Era, which is when this was. The life Expectency was 58 at best. During the Middle Ages not even that high for most of the populance. As a result marraiges happened much more early. Commonly marraige was not as odd as young as 14-16 as there was no generally higher levels of education, and society was by and large Rural and Agriculture based, with industrial working conditions that would make a modern human rights leader, or worker safety inspector die from Shock. The less you lived, the earlier you screwed. Its that simple and even if text is laking for those periods, the Tax man's records never have been. Got to get the gold out of those Peasants.
And this rp is not the show. Unless you see something I've missed. No one said you have to watch the show. Take a step back, be calm, and read.
The RP is about the bonds of masters and servents, with the later being demons. It has nothing from the canon. It is using the name to sell its self, and establish the norms in what Demons can do.
There is not one thing written in this RP set up, or in the show its self for that matter, about being adult men, playing with little boys genitelia. In the show there might have been bullshit fanservice, or fans implying, but nothing more than that.
In this RP its not the case at all, there are some sexual relations it seems, but for the time the ages so far fit that moral Standard. Plus we are talking about demons, as if they really care about morals. I mean they are Demons. And in any case human aging doesn't apply to them. For example you can have a demon that looks like a human Child but is 10's of thousands of years old. So sticking a biological legal age standard to them raises up all sort of legal question marks.
More so if a human dies and is reborn as a demon. Then inspite of being 28 apperance wise, they are 1 years old..as you can see if snowballs rather quickly.