Um, stranger things have happened but Mary being 60 while Samuel is 48 in the 1851 census, makes me a little suspicious. Especially as they have a 9 year old daughter making Mary 51 when she gave birth to her. Or perhaps Mary's year of birth is incorrect.
What evidence do you have that Mary and Samuel moved to Isleworth after their marriage in 1831?
They are obviously back in Northamptonshire in 1851. Daughter Charlotte was born in 1833 in Northamptonshire.
Do you have their marriage entry?
I couldn't read the 9 year old daughter's name but it is transcribed as Susan. I wonder if this is a hint that the child is named after her grandmother Susanna?

A submitted record on the IGI gives her name as Susanna and her chr as 13 Mar 1842 Benefield.
It is looking possible that the above family is correct.
I will try to find them on other censuses.