Typically when you find your ancestral family on the census sheet, valuable information on the ages and places of birth can lead you straight on to further targeted searching in the Old Parish Registers. But there are pitfalls. The enumerators who were employed to go round and enquire of each household who had resided in the house the night before simply wrote down what they heard, or thought they had heard. In an 1841 Census entry in New Monkland Parish the youngest of a Couk family was given as 1-year old Esebelau. Apart from spelling, deciphering of handwriting can be tricky.
Ages can be a bit unreliable.
The enumerator did not check birth certificates so was dependent on the honesty or memory of the householders. So a youngster might want to seem older, or an older person lose a year or two when asked, or a much older person genuinely forget exactly when or where they were born. Taken on 7th June Scotland-wide. Basic information on address, name, age, occupation and whether or not born in the County. Taken on 31st March Scotland-wide.