I know exactly what you mean, census data is rife with errors in spelling, ages, birth locations, you name it. There's no specific style guidance on this; I don't think we need it yet. Personally, unless the case is really egregious, I don't bother with an annotation - the text in the biography shows what the conclusions of the research are, and the sources generally make sense when you see the final product. In cases where it really helps to have a note, I just add it to the citation at the end - listed as Velsem instead of Wilson family, age mistranscribed by 20 years, whatever.