I agree. What I think is her death cert gives her age as younger (1836 birth year), so I take the 1831 with a pinch of salt, particularly as the 1871 census has Mary and Alfred Brownbell living with their son Alfred in Manchester. Admittedly Alfred is 10 when he should have been 6, but there was a lot of misunderstanding about rounding up ages I've been told. If it is them, her birth year is given as 1841, and she's said to be from Manchester - which implies she's been in Manchester for some while.
I know it's unlikely to be them, but having bought young Alfred's wedding cert, on which he said his name was Alfred Edward Ferns, and so was his dad's, I take anything they say with a pinch of salt.
Also the 1861 census answers would probably have been supplied by the landlady - and all her boarders are 30, apart from the children. I think she is probably rounding up.
If Lena existed - and I'm not too sure, seeing her nowhere else - then I reckon, like you, that Mary must have been younger than the censuses state.
Jo