I've often seen ages listed in fractions of a year 1/12, 2/12, etc. but it is not often that I see an age listed as a fraction of the number of days in the month. Maggie Johnson is listed as 20/30 for her age in this 1880 census enumeration from Prairie Township, Hancock County, Illinois (using the digital image at Ancestry.com)--it is indicated that she was born in May. The date this census was taken was 12 June 1880 and the actual date of the census was 1 June 1880. My guess is that this means she was 20 days old--but May does have 31 days, not 30.
Source:
1880 U S Census, Prairie Township, Hancock County, Illinois , page 20D, line 40.
