Hi Deb
Both John MELVILL( sic) her husband and she were stated as being 35
So as ages were rounded down in the '41 that makes both of them between 35-40 years old, so looks like Jane was born around 1801 - 1806 ish, if my arithmetic is correct (which it often isn't!

).
A qucik look on the IGI seems to come up with a few Jane McKays born in Sutherland during those years.
JEAN SCOBIE MACKAY - International Genealogical Index (submission)
Gender: Female Christening: 24 OCT 1800 Rogart, Sutherland, Scotland
Jane McKay - International Genealogical Index (submission, but says she married a Donald McDonald, so not yours)
Gender: Female Birth: About 1800 Tongue, Sutherland, Scotland
JEAN MACLEOD MAC KAY - International Genealogical Index (extract)
Gender: Female Christening: 20 SEP 1802 Durness, Sutherland, Scotland
JEAN MAC KAY - International Genealogical Index (submission)
Gender: Female Christening: 20 DEC 1802 Rogart, Sutherland, Scotland
JANE MACKAY - International Genealogical Index (submission)
Gender: Female Birth: 23 MAR 1803 New Year, Reay, Sutherland, Scotland
JEAN MAC KAY - International Genealogical Index (extract)
Gender: Female Christening: 21 DEC 1803 Durness, Sutherland, Scotland
Jean MacKay - International Genealogical Index (submission)
Gender: Female Christening: 21 NOV 1804 Of Mudale, , Sutherland, Scotland
Jean MacKay - International Genealogical Index (submission)
Gender: Female Birth: 21 NOV 1804 Mudale, , Sutherland, Scotland
JEAN MACKAY - International Genealogical Index (extract)
Gender: Female Christening: 20 AUG 1804 Farr, Sutherland, Scotland
JANE MACKAY - International Genealogical Index (submission)
Gender: Female Christening: 13 DEC 1805 Rogart, Sutherland, Scotland
JANE MACKAY - International Genealogical Index (extract)
Gender: Female Christening: 18 DEC 1805 Rogart, Sutherland, Scotland
JEAN MACKAY OR MACKAY - International Genealogical Index (extract)
Gender: Female Christening: 18 JUN 1805 Dornoch, Sutherland, Scotland
Jean Mackay - International Genealogical Index (submission)
Gender: Female Christening: 02 JUN 1806 Invernaver, Sutherland, Scotland
Jean McKay - International Genealogical Index (submission)
Gender: Female Christening: 29 SEP 1806 Grubmore, , Sutherland, Scotland
Jean Mackay - International Genealogical Index (submission)
Gender: Female Christening: 02 JUN 1806 Invernaver, Sutherland, Scotland
JEAN MC KAY - International Genealogical Index (extract)
Gender: Female Christening: 02 JUN 1806 Farr, Sutherland, Scotland
Which, unfortunatley, still leaves a lot of choices.... a lot of these seem to be submissions too...
Best wishes
Lesley