1871 Census records has him age 26 (born about 1845) from Hanover.
1881 Aged 36, at 45 Ascot St
1891 Aged 46, at 45 Ascot St

Name is confirmed on declaration of Doris's brith certificate, which also shows both spellings of Dittmer/Dettmer.

Marriage record has his full name as Johann Heinrich C Dittmer.

Cannot be the Johann Heinrich Dettmer, born 27 AUG 1847 in Wolthausen, Hannover, Preussen as he married Marie Dorothee Alwine Meine on 02 APR 1875 at Winsen, Schleswig-Holstein, Preussen.

Deaths Sep 1912   (50%)
DITTMER  John H C  67  Toxteth Park  8b 192

There's another John Henry Dittmer who died in 1902. This other John Henry Dittmer was born in Hamburg and lived in Kirkdale.
Deaths Dec 1902   (>99%)
Dittmer  John Henry  51  W. Derby  8b 262

Aged 30 in Apr-1871
Aged 32 in Sep-1872 (when died)


Therefore born:
After Apr-1840 (to be 30 on census)
before Sep 1840 (to be 32 when died)

Deaths Sep 1872   (>99%)
DITTMER  Dorothea  32  W. Derby  8b 276

Dorothea Dittmer
BIRTH    unknown
DEATH    14 Jul 1872
BURIAL    Anfield Cemetery and Crematorium
PLOT    Consecrated Sec.2, Grave 1261
MEMORIAL ID    174902970

Age: 32 years. "Death date" is burial date.

Born 13 or 30th or 31th July.  Birth certificate is diffucult to read.  However the 1839 register clearly states 30th July

Val Hawkyard (RootsWeb) has her as Catherine Dorothy Sophia (b.1868) which is confirmed by the birth certificate.
Her mother's name "Magdalene Dorothee" was mistakenly entered as the name and then corrected to "Catherine Dorothee Sophie".

"Doris" Aged 3 on 1871 census, born in Liverpool

DITTMER, CATHERINE  DOROTHEE SOPHIE     HEUER 
GRO Reference: 1868  S Quarter in LIVERPOOL 
Volume 08B  Page 30  Occasional Copy: A

Born at 199 Athol Street, Liverpool
Recorded at the Sub-District of St. Martin, Liverpool in the County of Lancaster.

An addendum to the birth certificate was made 58 years later:
In Entry No 55, in col 4 for "Christopher Dettmer" read "Johann Heinrich Christoph Dittmer otherwise Dettmer", and in col 5 for "Magdalene Dorothee Elizabeth Dettmer formerly Heuern" read "Magdalene Dorothee Elizabeth Dittmer otherwise Dettmer formerly Heuern".
Corrected on the 29th October 1926 be me Richard Mather, Suprt Registrar, on production of a statuory declaration made by Reginald Mitchell Banks and Maurice Davidson.

Probate:
DITTMER Catherine Dorothee Sophie otherwise Dorothea Catherine Sophia of 34 Bankburn-road Liverpool 13 spinster died 14 July 1942 Probate Liverpool 5 August to Charles August Dittmer shipwright.  Effects £212 9s. 4d.

Headstone reads:
Also Dorothea, / beloved daughter of the above / who died 14th July 1942, aged 73 years. 

Died in July 1870

DITTMER, GEORGE  FREDERICK CHRISTOPHER     HEUER 
GRO Reference: 1870  S Quarter in LIVERPOOL 
Volume 08B  Page 6

Deaths Sep 1870   (>99%)
Dittmer  George Frederick C  0  Liverpool  8b 4

Name    George Frederick Christopher Dittmer
Death Date    10 Jul 1870
Cemetery    Anfield Cemetery and Crematorium
Burial or Cremation Place    Anfield, Metropolitan Borough of Liverpool, Merseyside, England
Has Bio?    N

George Frederick Christopher Dittmer
BIRTH    unknown
DEATH    10 Jul 1870
BURIAL    Anfield Cemetery and Crematorium
Anfield, Metropolitan Borough of Liverpool, Merseyside, England
PLOT    Consecrated Sec.1, Grave 1176
MEMORIAL ID    174902971