There is certainly a lot of nonsense on some of the trees for this family on Ancestry.Robert's immigration entitlement certificate gives the following information:
Ship: Neptune, Arr. Feb 1844
Robert FISHLOCK Native of Devizes, Wiltshire Parents' names: Roger and Mary - deceased Calling: Farm Labourer Age:44 Baptised: 1 May 1803 Religion: Episcopalian Can Read
Elizabeth FISHLOCK Native of Devizes Parents: William and Anne AMBER or AMBOR (Farm Labourer) Calling:Farm Servant Age:42 Baptised 1 November 1801 Religion: Episcopalian Can Read
Michael, 13, can read, baptised 28 March 1830 James, 7, can read, baptised 24 April 1836 Thomas, 4, baptised 20 Oct 1839 Eliza,9, can read, baptised 13 April 1834
The baptisms were certified by the Bishops Cannings parish priest.
Given this information, this would be Robert's baptism from FamilySearch:
Name: Robert Fishlock gender: Male baptism/christening date: 01 May 1803 baptism/christening place:BISHOPS CANNINGS,WILTSHIRE,ENGLAND mother's name: Mary Fishlock indexing project (batch) number:P01438-1 system origin: England-ODM source film number:942.31 B2 V26B
This baptism with no father's name suggests that Robert was illegitimate and was probably aged 3 or 4.Perhaps his mother later married a man named Roger, or perhaps he didn't exist. You really need to see the original entry to find what information it provides, but I don't think it will give you a date of birth otherwise that would be included on the index.