Looks like Vexell has you sorted.
If you want a specific age, though. Futoshi route she seems to range from 10-11. Manga she is 12.
None of the doujins are a hard canon to the manga except the Master's First Time. Take everything else with a grain of salt. A rough guide at best, as Hagu has changed her past and actions multiple times. Also things marked with "IFIFIFIFIFIF" are exclusively non-canon.
I disagree at this point. By now there's almost a full year worth of posts and warnings. That poster clearly stated he didn't even bother looking before jumping in, then gets mad. Fuck'em.
Sure, it's bound to happen that some people don't check out the comments, I'm saying that some others will, and skip straight to the most recent ones. For those specifically, periodic reminders can serve as useful warnings.
We have to consider that some people would not have prior knowledge of the author, and think it's another kid/ teenager romance kinda in the vein of takagi san, for example. Those would not have much cause to be suspicious, aside from the "suggestive" tag. I think the description is somewhat lacking as is, and could serve as a forewarning, or to direct potential viewers towards the comments to get informed before taking the plunge.
Also, some people would purposefully not go through comments to avoid spoilers, I think.
That this manga exist at all is baffling to me (no judgment, though), and I can see where the previous complainer is coming from. If it was a separate reality remote from the doujins, it'd be one thing (though still weird to use this specific character), but now, intentionally or not it serves as a funnel towards the hardcore stuff that those unsuspecting are not ready for, nor have they signed for that kind of content when they first clicked (I assume). The begining being relatively tame would ease them into it, and the reveals would then be felt as if they had been deceived all along.
All in all, I think it's fair for people not to know, and crash out when they find out. Then they come here to vent. It's fine, too, but here you'll find mostly two types: those that come from the hentai, and are fully on board with what's going on, and those that are revolted and promptly got out of here, never to return. Which is to say, you'll only get replies from the degenerate faction, since this is mostly their fiefdom.
Folks coming here to complain should do it aware that they're screaming into the void and that nothing will change, that's my point. I'd welcome an intelligent discussion on the morality of this manga existing at all, but those comments are written on an emotional impulse, so I don't hold my breath for that happening.