Some would argue that she was not too old
The last child (William) on the CR could easily have been born when she was 50/51. Mary Ann could easily have been his second wife or perhaps he started late.
I think KGarrard has found the right family for you - spelling is just a matter of how they said their name to the enumerator & how he interpreted what they said.
A copy of William's birth cert will give you all the info you need inc mothers maiden name but there are 2 potential marriages on FreeBMD that might fit the bill.
John SCARF & Mary Ann REED MarQ 1857 Sudbury Ref 4a/509. Sudbury RD covers the villages just west of E Bergholt.
John SCARFE & on the same page Mary Ann GURLING Cosford RD Dec 1850 12/667. Cosford RD is more NW of E Bergholt.
3rd option is he could have nipped across the river into Essex & married there - Bradfield is not far across stream from E Bergholt
The John SCARFE death reg you have found in the Samford RD is likely this one
John SCARFE age 76 buried 25 May 1879 Chelmondiston, St Andrew (Suffolk Burial Index).
I found a Mary Ann SCARFE buried 18 Jul 1900 East Bergholt age given as 78. If it is the same Mary Ann then she could have been younger than the 1871CR suggests or her age at death was a guesstimate. Samford RD SepQ 1900 4a/653. No sign of husband John buried at E Bergholt up to 1900 on SBI
K
