Hiya, sorry, I wasn't ignoring you, I was off being banned for a day for being unfriendly, and then I still couldn't get back in ...
I was just going to say, about William's various ages, to remember that he might not have been baptised at birth, so an 1814 baptism and an 1812ish birth are still possible. Just plain negligence in age reporting is always a good option too.

The marriage certificate should help you along a fair bit.
Just to note that when you search for Tabb baptisms in Merther and nearby parishes, there is a series in St Clement:
Jane 1820
Mary Anne Grigg 1822
Eliza 1825
and John 1830 St Enoder
(I include John from checking the 1841 census)
That William and Mary aged 65 and 50 are with children Eliza 15 and John 10 in St Enoder in 1841 but seem to be deceased by 1851.
Given the age difference ... there's an Elizabeth Tabb baptism in 1808 in Merther with parents William and Ann ... could Mary have been a second wife?
Just idle poking around ...