Enumerators were instructed to round ages on the 1841 census down to the nearest 5 years for everyone over the age of 15, although not all enumerators carried this out. Therefore all people aged 15 to 19 should have been recorded as being 15 years old. By the same reasonning the computed date of birth given by most organisations using these records is 1841 minus 15 years ie 1826. I expect that John and Elizabeth ages were within the 15-19 bracket and therefore probably not twins. Usually the eldest is listed first although sometimes males were listed before females.