I very much doubt that William Ninnes would have been married at age 12 for a start!
So I reckon you can ignore that little piece of information.
Surviving Bishops Transcripts for Towednack show no record of a baptism and the earliest surviving Parish Register for Towednack does not commence until 1676 so any one claiming a birthdate for William is guessing.
William son of William and Jane Ninnes was baptized at Towednack in 1692 and he pre-deceased wife Ann as per the following burials at Towednack:-
William Ninnes buried 15th March 1767
Ann Ninnes, widow, buried 12th February 1771
William Nines and Ann White married at Gwithian 17th February 1723 with the following children:-
Thomas bp. 27th November 1724 Towednack
William bp. 6th January 1726 Towednack (married Catherine Cock 9th November 1753 St Gluvias)
George bp. 3rd January 1730 Towednack ** see below **
Ann bp. 17th February 1733 Towednack and buried 12th April 1754 Towednack
Jane bp. 25th February 1736 Towednack and buried 4th March 1736 Towednack
Thomas bp. 26th August 1739 Towednack (married Mary White 1st April 1766 Towednack)
George Curnow was married twice but (as with William Ninnes above) I very much doubt that Ann Jelbart would have been married at age 13!!! So any information about her being born/baptized in 1758 I should think is another fantasy of someone!
In fact after looking at the information again I should think the details you have for Jane White might also be wrong. There is no Jane White baptized at Towednack until 1831, over 100 years later! Work I did a few years ago indicates that Jane White was baptized at Phillack 6th January 1745 daughter of George White and Anne Canarthen.
Returning to Ann Jelbert - a list of possibilities can be made based on the baptisms of her children but we need to take into account the ten year gap between daughters Mary in 1781 and Elizabeth in 1791. If Elizabeth was born in 1790 or 1791 then the earliest Ann is likely to have been born would be about 1742 but if she were born closer to her elder sister then we might look as early as about 1734. The latest I think she would have been born would be about 1754 but realistically I think 1752 would be more likely. Also take into account the variations such as Jelbart, Jelbard, Jelbert, Jelberd, Jilbert, Jilbart, Gilbart, Gilbert and the list I find is this:-
Anne daughter of John and Anne Jelbard (BTs) bp. 5th November 1746 St Just (surname is Gilbard in the Parish Register)
That's it - just one possibility is all I can find. There is always the possibility there may have been another Ann for whom a baptismal entry is missing or perhaps never recorded but it does look like this will be the girl. She would have been about 24 or 25 at marriage and about 45 at the baptism of Elizabeth in 1791.
To be more certain of this however it would be an idea to check for possible burials or other marriages for Anne.