I read couple of definitions of sunnah and they were all something like "whatever the Prophet (pbuh) said, did or approved to do." But none mentions these have to be after Islam have been revealed to him (pbuh). On the other hand, I don't recall anything called sunnah that happened before Islam. Is there an implied "after Islam revealed" in the definition of sunnah?
The quran simply says to follow the prophets example, it dosent say to exclude any certain point of time such as the time before he became a prophet
Say, [O Muhammad], "O mankind, indeed I am the Messenger of Allah to you all, [from Him] to whom belongs the dominion of the heavens and the earth. There is no deity except Him; He gives life and causes death." So believe in Allah and His Messenger, the unlettered prophet, who believes in Allah and His words, and follow him that you may be guided.7:158
"… He [Heraclius] said, ‘Had you ever blamed him for telling lies before he claimed what he claimed?’ I replied, ‘No.’ … He said, ‘Does he break his promises?’ I replied, ‘No, but we are now at truce with him and we are afraid that he may betray us.’ … . ‘When I asked you whether he was ever blamed for telling lies, your reply was in the negative, so I took it for granted that a person who did not tell a lie about (others) the people could never tell a lie about Allah. I asked you whether he had ever broken his promise. You replied in the negative. And such are the apostles; they never break their promises…’" (Sahih al-Bukhari, Volume 4, Book 52, Number 191)
The hadith above exlains that even the enemies of the prophet knew that he never told a lie, before and after prophet hood, demonstrating that he was a role model before he bacame the prophet.
Furthermore, that during the isra wal mi'raj the Prophet (pbuh) was offered wine, water and milk as a drink by Jibreel (or gabriel) The prophet (pbuh) chose to drink milk upon which Jibreel alaihi salaam commented: you are rightly guided and your Ummah will be as well.
I would also like to point out that the incident explained above occured before alcohol was made unlawful, so the prophet was a guide to his ummah before Allah commanded much of the the prohabition of certain things.
This shows that the prophet was rightly guided before he had the title of the prophet, and we are simply told to follow him to be guided, Allah did not differentiate before or after he became a prophet, as he was a good example at all times.
Consider this: the Prophet (salallahu alayhi wa sallam) was sent as a Prophet, to guide us to the straight path. That was the mission statement of his life after prophethood.
As Muslims, being told to follow his sunnah, obviously means that we should follow whatever Allah inspired him to, as part of his prophethood. Also, as a prophet, Allah protected his actions from anything wrong -- like in Surah Abasa, when he turned away from Abdullah ibn Umm Maktoom, Allah revealed a rebuke in the Qur'an.
Also, sunnah, linguistically, means something habitually done by someone. Islamically, it means a collection of actions, approvals, etc. that Allah inspired the Prophet to do.
Based on these points, it doesn't really make sense to follow his actions prior to his prophethood as part of his sunnah, because they are not linked to divine guidance.
Source: Seerah + usool-ul-fiqh classes
There are certainly stories form before Muhammad (Peace and Blessings be upon him), became a Prophet of Allah that we can learn form and are often cited as such, though I do not have a grasp as to whether these have the same level of authenticity as when his actions were being actively recorded.
One such story is told by Muhammad himself:
The apostle of Allah later told how Allah had preserved him in his childhood and state of innocence, saying, ‘I was among Quraysh boys and we were carrying stones for some play. We were all naked, and every boy had placed his ezar [loincloth] around his neck to carry stones in it, and I had done the same as . When I was thus moving about, some Being whom I did see struck me a fearful blow and exclaimed, “Bind on thy ezar “; accordinglyl I girded myself therewith, and thus carried the stones on my neck, I being the only one among my companions who wore his ezar.’