Of the ones we know for sure, Sinclair is 20, Yi Sang is 27, Gregor is 35, and Don is >200. However Limbus does have more time progression than is immediately apparent, so they could be slightly older by now. Late 20s for some others wouldn't be that wild to me though.
As for why Dante (and Moses for that matter, since we know she's currently working with Limbus Company) couldn't detect anything wrong, given that the other bloodfiend didn't know untill she transformed eirher I believe that however the distortion is suppressed is rather thorough about it. Don does want to hide that aspect of her, so it makes sense that the method picked would provide some defense against more esoteric methods.