In the 1861 census, there are two families of young George Dennes, first:

Name Relation Condition Sex Age Birth Year Occupation   Disability  Where Born

DENNE JUNR, William Head Married M 43 1818 Farmer 150 Acres Employing 12 Men 3 Boys and also Builder Employing 40men & 4 boys   Walmer, Kent
DENNE, Mary Ann Wife Married F 42 1819   Rochester, Kent
DENNE, Mary Daughter Unmarried F 18 1843   
Walmer, Kent
DENNE, Ann Daughter F 15 1846   Walmer, Kent
DENNE, Harriet Daughter F 11 1850 Scholar   Walmer, Kent
DENNE, Thomas Jull Son M 10 1851 Scholar  Walmer, Kent
DENNE, Sophia M Daughter F 8 1853 Scholar  Walmer, Kent
DENNE, George Son M 5 1856 Scholar   Walmer, Kent
DENNE, Sarah Daughter F 3 1858 Scholar   Kent
MANSER, Susana Visitor Unmarried F 17 1844 Farmers Daughter   Kent
GOODBAN, Mary J Servant Unmarried F 23 1838 Dairy Maid   Ringwould, Kent
PAUL, Eliza J Servant F 16 1845 House Maid   Deal, Kent

RG number: RG09 Piece: 544 Folio: 13 Page: 19    

Registration District: Eastry Sub District: 4 Deal EnumerationDistrict: 9
Ecclesiastical Parish:

Civil Parish: St Mary Municipal Borough:

Address: 88 High Street, St Mary, Village Walmer
County: Kent
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Here is the second census:

Name Relation Condition Sex Age Birth Year Occupation   Disability  Where Born

DENNE, George H Head Married M 39 1822 Powder Maker Employing 30 Men 2 Boys   Walmer,
Kent
DENNE, Mary A Wife Married F 34 1827   Dover, Kent
DENNE, Mary M Daughter F 8 1853   Deal, Kent
DENNE, George H Son M 4 1857   Deal, Kent
BROWN, Emily Servant Unmarried F 14 1847 House Servant   Deal, Kent

RG number: RG09 Piece: 543 Folio: 9 Page: 12    

Registration District: Eastry Sub District: 4 Deal EnumerationDistrict: 1
Ecclesiastical Parish: St Leonards

Civil Parish: Deal Municipal Borough: Deal
Address: 4, Lower Street, Deal
County: Kent
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And a close examination says that it is the second that had to be him, for two reasons that there is a H in the forenames and that Deal is clearly given as the birth place.

If this is now accepted, note that the father, George H Denne says he was born in Walmer.
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There are also two birth records for these Georges:

Surname First name(s) District Vol Page

Births Mar 1856
DENNE George Eastry 2a 661   
Births Jun 1857
Denne George Henry Eastry 2a 609

This confirms the censuses with the younger age for George Henry.  Perhaps the birth certificate should be purchased to confirm his parents but I’m not going to that expense for non-ancesotrs!
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Their marriage cannot be found on FreeBMD.

Here’s their marriage on FreeBMD:

Surname First name(s) District Vol Page
Marriages Dec 1890
DENNE George Henry Eastry 2a 1773   
Gardner Nellie Ralph Eastry 2a 1773   
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