Hi Shirley,
Yes your right to have misgivings and its early days.
The age on these post 1837death certs taken as must be right as gov.
My Gt Granny was 20 at marriage 1867 & died aged 60 1902
and signed X thus but was educated at counting in 10's and not in halves 0.5 -50% or dived by 2.=1/2 or 5years round offs
My oldest known on my mums side Phoebe Tidmarsh died 1852 age 83 on death cert- on the 41c age 60 on the 51c age 72 buried at Temple Balsall churchyard on her head stone age 72.
As siblings of 1787 Bolsover are in hugh wallis bolsover
http://freepages.genealogy.rootsweb.ancestry.com/~hughwallis/IGIBatchNumbers/CountyDerby_(A-M).htm#PageTitleJohn could be problems 1793 could have died as infant & 1807 2nd John child then moved on after last entry
If she was married before then you've a life time work. but ! think not.
Another marriage still out there round 1797-1806 but can't rule out 1787
the normal marrying age . = 1785-1795 of 20 to 30 years old at marriage
5 years diff at death age ~ marriage years out by + 10 years to the norm
funny old world
I'll confirm Eiz marsh 1771 bapt smalley
good hunting
Doddy