I've looked again at 1841, 1851 & 1861 census and noticed a couple of things.
1851 Census
Address 17 Doris Street
John Johnson, estimated year of birth 1807
Mary Johnson, wife, estimated year of birth 1804; place of birth Blackfriars, Surrey
John son age 18 POB Lambeth
Ann Henrietta Johnson, daughter, age 13, estimated YOB 1838; POB Birmingham
Richard son age 10 POB Lambeth
Maria daughter age 8 POB Lambeth
Ellen daughter age 6; estimated year of birth 1845; POB Lambeth
1861 census 17 Doris St.
John Johnson, estimated year of birth 1807
Mary M. Johnson, wife estimated year of birth 1808; place of birth Christchurch, Surrey
Richard C. son age 20 est. year of birth 1841 POB Lambeth
Maria daughter age 18 est. yr. of birth 1843; POB Lambeth
Ellen daughter age 15 estimated year of birth 1846; POB Lambeth
1841 census
Household consisted of adults, John & Mary Johnson (both aged 30 giving estimated YOB 1811) + 4 boys including Richard aged 1. Ann was not present.
Observations:
1.Ellen's was recorded as age 6 on 1851 census and age 15 on 1861 census. If those ages were both correct it would indicate a birthday in March or April. When were dates of 1851 & 1861 census?
2.Mary has a middle initial M on 1861 census. Mary's estimated year of birth varied by 4 years between 1851 & 1861 census. Her stated place of birth also differs slightly. (My knowledge of geography there is sketchy.) John's age was consistent. It may simply have been a case of a woman wanting to believe she was younger than she was.
3. Ann appeared with the family on 1851 census only. She would have been around 3 years old at time of 1841 census. Her place of birth was Birmingham. Elder and younger siblings all had POB Lambeth.
Ann Johnson birth registration to consider:
1837 D quarter Birmingham; Ann Harriet Johnson; mother's maiden name Johnson.
Ann's middle name in 1851 census enumerator's book was Henrietta but it may have been an incorrect transcription of Harriet.
Alternatively Ann Henrietta Johnson may have been from a different relationship.