Elizabeth McCartney's birth is showing as a submitted entry on IGI as 1843 in Edinburgh so age above would fit.
Added: The 1851 census for Elizabeth I would think from the details you found Tracey on the MC:
John McCartney 35, Costermonger, b. Ireland
Helen McCartney 29, b. Ireland
Steven McCartney 13, b. Ireland
Elizabeth McCartney 9, b. Ireland
Helen McCartney 8 Months, b. Edinburgh
Address: Maconnochies Close 44 Cowgate, Edinburgh St John
Costermonger was a Street seller of apples, other fruit and vegetables
www.scotsfamily.com/occupations.htmLuckydog, there is an 1855 birth cert showing for a child to John and Helen on IGI:
MARY MC CARTNEY Birth: 23 NOV 1855 Edinburgh Parish, Edinburgh
1855 was the first year of official registration in Scotland and the certs ran over to two pages on the register. For births, it should include date and place of parents' marriage, ages, occupation, where the parents were from and how many children they'd had (alive or deceased). A good one for you to look at if you haven't already. With Irish families, it may be the only clue as to where they came from in Ireland
