I have just had a look at the original image for the 1871 Census and I can tell you that Maria was age
58 and not 68.
On first look it most certainly looks like 68 but enlarging the image a little shows that it is, in fact, 58.
I suspect therefore that John may have been only 67.
With Maria being 58 in 1871 her birth should have been about 1812 or 1813.
There was a Maria CURTIS d/o William (mariner) and Mary of Ventonleague baptised at Phillack 20th November 1814 so the information you found 'might' be correct.
I will try and find the marriage which will give us further clues.
In the 1861 Census John Penberthy is 'abroad' and Maria and the children are living at Guildford, Phillack.
Eldest daughter living at home was Eliza who was then age 24 and recorded as being born at London.
Next child was daughter Maria age 19 who was born at Bristol, Gloucestershire.
Then comes Anna born at Phillack about 1844.
Ellen born at Bristol about 1846.
Angelina born at St Gluvias about 1851.
Sarah born at St Gluvias about 1854.
Mary J born at Phillack late 1860 or early 1861.
I am so far unable to find the family in 1851 however 1841 might be interesting.
There was a John Penberthy age 30, iron founder, at the Bristol Infirmary.
John was not born in Gloucestershire and it is quite likely he was actually born in Cornwall.
His age suggests a birth between 1807 and 1811 but I would not take too much notice of that just yet.
Based on the family in 1861 we should be looking for a marriage about 1835 or 1836.
Further of interest from 1841:-
At Lower Cottage Street, St Augustine the Less, Bristol
Maria Penberthy, 25, Ind., Not born in County
Mary do., 6, Not born in County
This 'could' be the wife of John and with daughter Mary age 6 would suggest a marriage about 1833 or 1834.
But if this is the right person then where was her daughter Eliza at this time?
As the LDS site is down for maintenance (still) I am unable to do any more on this problem for now.
A few things to think about anyway.