A little bit more! The record of the burial of the Peter Pegg who died in 1918 (my great grandfather) shows him as being born in 1843.
The birth certificates of his children were signed by him with his mark (x) so he must have supplied his age in each case.
If Peter Pegg senior was drowned in 1845 or 46, it could be that Peter Pegg junior did not properly remember his age and that he may have been a year younger than he thought. Later records may derive his age and date of birth from his earlier estimates.
He certainly could not have been the child of Peter Pegg and Sarah if he were born on the Clarence in 1843 and they arrived in Sydney 15 March 1844. It is just possible he could have been born in 1844.
This does not explain why two siblings would have the same first name. I note that Peter Henry is the only child with a middle name. Does this pose another question?