1851 gives his birthyear as 1811 but the 1871 shows 1806. Can't find an 1861 entry at the moment but as son Charles was b Spain 1856 - that may be why
Sorry, Carole, but you have woken up one of the bees in my bonnet!
Some transcription or other may say that he was born in 1811. The original census
never states a year of birth. It states the age the person said they were on the day of the census.
Many transcriptions do not stick to what is actually on the original census document. They attempt to calculate the year of birth by subtracting the age from the census year. This gives the
wrong year three times out of four.
Why? Because from 1851 onwards the census was taken around the end of March or beginning of April. This is one quarter of the way through the year. By this time only about a quarter of people have had their birthdays since the beginning of the year. The other three-quarters will have a birthday before the end of the year, so they will be another year older by the end of the year.
The 1871 census was taken on 2 April. Assuming that John said he was 65, and that he was correct, he would therefore have been born between 3 April 1805 and 2 April 1806. If he said he was 40 in 1851, he would have been born between 31 March 1810 and 30 March 1811.
Clearly, these dates do not overlap so he must have got his own age wrong in at least one census!