Muslim scholars are not unanimous about the Esmat of the Prophets before their prophet-hood. Majority of Sunni Muslims scholars do not believe in the Esmat of the Prophets before their prophet-hood. Shi'a Muslims by contrast regard the Prophets immune from sin from birth to death whether since or before their prophet-hood.
Apart from the guidance of the Imams of Ahlul-bait (a.s) the main reasoning of Shi'a is based on logic and intelligence.
They argue that if there has been any sinful or criminal record in the history of a prophet, he will lose the trust of his people. Public acceptance depends largely on the credibility of the prophet. Thus, the Prophet of Islam (saww) was known to his community prior to his mission as ‘Mohammad the Trustworthy’.
In our day to day life, for example, if two people are nominated for the office of the leader of a mosque or a church and are equal in all necessary conditions save one of them who has a past criminal record and the other does not, common sense commands to vote for the one without any negative record. Today we observe the fact, that even to be accepted in a simple position such as ‘Justice of the Peace’ not only a criminal history, but even a traffic offence is an issue for the applicant. Then, how could God the Almighty possibly assign a man with a sinful record for the holy office of prophet-hood.
Shi'a scholars even assert that a prophet has to be the most intelligent and healthiest person with no physical disability, and a handsome person etc. all which play a positive role in their mission. God, therefore, selects the best with His might and wisdom.