My turn to try and add something to this little puzzle!
I think Cornishmaid has picked up another person here but there is probably a connection anyway.
You say that your Johnson was aged 5 in the 1841 Census so he is born c.1836.
In 1851 he is at home with his parents and was then aged 15 and born at Gwithian.
Father James was aged 51, bn. Camborne and mother Mary was then 53 and born at Gwinear.
Other members of this family in 1851 were Ann (22) and Jenefor (13) who were both noted as having been born in Gwithian.
Now - there is some conflict here about the baptism of James Rule at Camborne in 1800.
According to Zenobia the parents named at the 27 July baptism were JOHN & MARY.
But what I have found for that same date (Cornwall Online Parish Registers) is that the parents were JOHN & ELIZABETH.
I am not sure where Zenobia got her information from but I have just checked IGI and it also gives the parents of James at Camborne in 1800 as John & MARY.
The one problem with IGI is that it also has two entries for 27 Jun 1800 for the same person as well as the JULY date.
Parents appear to be definitely John & ELIZABETH.
In that case we are looking at the following marriage at CAMBORNE:-
20 September 1794 - John RULE, miner m. Elizabeth THOMAS (wit:- James Thomas and Richard Thomas)
Children:-
Jane bp. 26 July 1795
Stephen bp. 24 June 1798
James bp. 27 July 1800
John bp. 31 January 1802
Mary bp. 17 October 1802
Elizabeth bp. 08 January 1804
Elizabeth bp. 14 January 1805
Clarinda bp. 09 May 1807
William bp. 07 May 1808
Mary Anne bp. 26 December 1810
Joseph bp. 08 January 1814 (of Gwithian)
William bp. 30 March 1816 (of Gwithian)
It is 'possible' there may be another John and Elizabeth Rule involved here but I have not yet found a marriage.
What I now need to do is check some Camborne Burials that might fit into the above.
Other than that - I think you will find that John and Johnson may have been mixed up a little in a few entries - but I will look into that.
Best post this now as I am having a few little computer problems and I don't want to lose this information.
Jemima bp. 29 Aug 1818 (of Gwithian)