I have someone who appears twice on the 1861 census with different ages and places of birth but is definitely the same person.
Here are all the details for her - Emma Isted
birth ref - 1846 Brentford Registration district
baptism - Nov 1848 Twickenham - this is a couple of months after her younger sister was baptised ( why not have them baptised together
1851 census Twickenham
James Isted age 29 born Buxted, Sussex
Mary Ann Isted age 29 born Buxted, Sussex
James Isted age 11 born Buxted, Sussex
Thomas Isted age 9 born Twickenham, Middx
Charles Isted age 7 born Twickenham, Middx
Emma Isted age 5 born Twickenham, Middx
Mary Isted age 3 born Twickenham, Middx
so age and place of birth would fit with birth ref
1861 Twickenham
Emma Isted Lodger age 15 bornTwickenham, Middx
Henry Isted Nursechild age 4 born Twickenham, Middx
so age and place of birth consistent. Henry is her brother. Mother has died and father remarried
but she appears on 1861 census with her father and family with different age and place of birth
1861 Richmond
James Isted age 40 born Buxted
Anne Isted age 39 born Twickenham
Emma Isted age 12 born Kennington
Mary Ann Isted age 11 born Twickenham
George Isted age 9 born Twickenham
Kennington is not Brentford district
1871
James Isted 51yrs b Buxted
Ann 48yrs b Twickenham, Middlesex
Emma 21yrs (single) b Kennington, Surrey
Henry 13yrs b Twickenham Scholar
can't find in 1881 - not with her father
1891
James Isted age 72 - no place of birth listed
Wife Anne age 66 - born Twickenham
Daughter Emma age 42 - born Kennington, London
she died Jan1899 age 51
so all the later census put her as being born 1848 but her sister was born then. I can understand women lying about their ages later on in life ( her sisters ages fluctuate too) but why at the age of 15 would they knock a few years of her age ? Maybe it was the stepmother giving the info.