a claimed age of 40 in 1901 = a claimed birthyear of 1861 give or take a year, ie some 5 years older than the John Thomas discussed above.
Reported ages in censuses are generally quite accurate for children but often get increasingly more inaccurate as the person ages. Early marriages, wide gaps in ages between spouses etc are factors that encourage inaccuracy in reported ages and should lead to increased suspicion in reviewing the evidence.

Women usually 'disappear' from the record by changing their name on marriage - if she gave up a child for adoption on the Island I would have thought it most likely she then left the Island - however the task of finding her in the 50M of the UK (assuming she went no further) given the 100 year rule on most personal data is not going to be easy unless she kept in touch with some family member.