This looks to be the family's 1841 census entry. This census is the most limited in terms of info. Ages were supposed to be rounded down to the nearest five years for everyone over 15 and no relationships within the household are shown:
Thomas Duncan 30, smith, b. Scotland
Ann Duncan 30, b. Scotland
Jane Duncan 6, b. Scotland
David Duncan 4, b. born in the County
Robert Duncan 13, b. born in the County
John Duncan 11, b. born in the County
Address:Lethenday Old, Scone Perthshire
This entry comes from a trascript which is not always accurate in terms of transcription

I don't know for example whether the last two, Robert and John, are actually that age or the age is showing in error. For example, Robert should show in 1841, aged c. 1. John may also be one of their children with age mis-transcribed or a relative. If you look at the original it will hopefully be clearer.
Ann's age above looks light (anywhere from 1807-11). However, the 1841 census is the most unreliable in terms of age and you need really a couple of censuses, if you are going by these alone, to get a better feel for someones age.
The good thing is that Thomas father is showing in the 1861 census, which means that his death cert, if he died in Scotland, should be available to view on Scotlands People, and will confirm his parents' details. This is his 1861 entry:
Thomas Duncan 49, blacksmith, b. Strathmiglo, Fifeshire
Margaret Duncan 17, weaver, b. Scone, Perthshire *
Thomas Duncan 15, shoemaker, b. Kinross
Address: Riggs Row, Cupar Fife
*I think this may the Maryann that showed in 1851, again hard to say what is actually her name without looking at the original images.
Monica