Could it be a case of Mary knocking a few years off of her age? She appears on the 1841 census HO107 piece 423 book 3 folio 9 page 14 at Lower Moor, Eardisley, with James and she is age 50 b1791 - 10 years later she is just 51!
There is a marriage in Llanstephan, Radnorshire of a James Meredith to Mary Powell on 16/7/1814 (could it be a possible? you know how bad my geography is!) If so Mary would have been older than what she declared on the census from 1851 onwards.
Perhaps I was getting too carried away trying to prove Mary was over 100 This looks a more likely marriage James Meredith to Mary Griffiths 17/5/1826 in Knighton