In her book "The Indomitable Mary Easton Sibley" Kristie C. Wolferman gives an account of the loss suffered by this family in the year 1847-1848 on p. 120. She gives Mary's age as "abt.6 yrs. old" which is more plausible as her mother had died in March 1847. The headstone is difficult to read but you can see the 6 mos and 1 day script. Benjamin remarried in Dec 1847 so it remains unlikely that she was only 6 months old when she died. The majority of the Alderson's were first buried at Lindenwood and then moved to the present location. Old headstones may have been recut with incorrect information.
It is possible, not proven, that her date of birth was 8 Oct 1841.